Top 40 Interview Questions on Docker for Professionals

Interview Questions on Docker for Professionals

Docker remains a highly in-demand technology in the current job market due to the need of containerization, microservice architecture, DevOps and CI/CD implementation, cloud computing, and so on. We cover sought-after skills such as basics, compose, networking, security, and orchestration throughout this blog that contains top 40 interview questions on Docker. Explore the fundamentals in our Docker course syllabus.

Docker Interview Questions and Answers for Freshers

Here are 40 basic Docker interview questions and answers that cover a range of topics:

1. What is Docker?

Applications inside containers can be automatically deployed, scaled, and managed with the help of the open-source Docker platform. Applications may operate reliably across several systems thanks to containers, which offer an isolated environment.

2. Differentiate a container and a virtual machine.

Virtual Machines: Make the whole operating system virtual. The guest operating system for each virtual machine might be resource-intensive.

Containers: Virtualize the application layer with containers. They are lighter and more effective since they share the host OS kernel.

3. What are Docker images?

Docker images are documents that include the setup instructions for a Docker container. The files, libraries, and dependencies required to run an application in a Docker container are contained in read-only Docker images. They are comparable to virtual machine environments’ snapshots.  

The application code, libraries, dependencies, and configurations required to operate an application are all included in read-only templates called Docker images.

4. Define Docker Containers.

Docker images are instances of Docker containers that run at runtime. They can be begun, paused, and erased. They are made using photographs.

5. Describe a Dockerfile.

A text document with instructions for creating a Docker image is called a Dockerfile. The base image, commands to execute, and files to copy are specified.

6. Describe the Docker Hub.

Docker images can be found and shared via the public registry known as Docker Hub. Additionally, it offers private repositories where you can keep your own photos. 

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7. What function does the Docker daemon serve?

The host computer runs the Docker daemon as a background service. Docker containers are built, executed, and maintained by it.

8. Define Docker Client

A command line interface (CLI) called the Docker client enables users to communicate with Docker. 

With the Docker client, users can:

• Take pictures out of a register
• Use a Docker host to run images.
• Construct and oversee networks, containers, and Docker images.  

You can communicate with the Docker daemon using the Docker client, which is a REST API or command-line interface (CLI).

9. Explain the concept of Docker registries.

A centralized mechanism for distributing and storing Docker images is called a Docker registry. Repositories for storing and sharing Docker images are called Docker registries. One well-known public registry is Docker Hub.

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10. Write the command to build a Docker image

The command to build a Docker image is docker build -t <image_name>:<tag>.

11. How do you run a Docker container?

docker run -it <image_name>:<tag> using this command, we can execute a Docker container.

12. How do you list running containers?

We can list the running containers using, docker ps command.

13. How can all containers, even those that have been stopped be listed?

It can be done with this following command:

docker ps -a

14. How do you stop a running container?

The running container can be stopped using, docker stop <container_id>.

15. How do you remove a container?

A container can be removed using, 

docker rm <container_id>.

16. How do you remove an image?

docker rmi <image_name>:<tag>

This above command is used to remove an image.

17. Explain Docker Networks.

Users can link Docker containers, services, and components to external components and workloads as well as to one another using the Docker networking feature.

• Communication: Communication between containers, between containers and the Docker host, and external internet access are all managed via Docker networking. 
• Isolation: For Docker containers, Docker networks offer total isolation. 

18. Explain different types of Docker networks.

Three typical Docker network types are as follows:

• Bridge Networks: It is utilized by a single host.
• Overlay Networks: For communication between many hosts
• Macvlan Networks: used to directly connect host network interfaces to Docker containers  
• Host Network: Containers share the host machine’s network stack.

19. What are the drivers for networks?

Five network drivers that implement essential networking capabilities are pre-installed in Docker.

Network management: Subcommands allow users to connect, detach, remove, prune, list, establish, and investigate networks.  

20. What are Docker volumes?

A feature called Docker volumes enables users to manage and store data for containers in a persistent manner. Containers are persistently stored in Docker volumes. Volume-stored data is not lost upon removal of the container. 

Storage: Data can be stored outside of containers using Docker volumes. They are kept in a directory on the Docker host and are generated and controlled by Docker.

Persistence: As volumes are persistent, data is kept on them even after the containers that use them are deleted.

Sharing: The suggested method for sharing data between several containers is to use sharing volumes.

Performance: The raw file performance offered by Performance Volumes is identical to that of direct host filesystem access.

Use cases: For long-term storage requirements and performance-critical data processing, volumes are perfect.  

21. What are the benefits of using Docker volumes?

• The docker volume create command allows you to explicitly create a volume.
• The volume can be mounted into a container.
• With manual tools, you can directly access the volume.
• To save volume data in various services, you might use multiple drivers.
• To mount volumes into containers, use the -v and –mount options.
• You can give a volume a unique name by using the name field.
• The external feature allows you to specify the name that will be used to search the platform for the actual volume.

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Interview Questions on Docker for Experienced Professionals

22. Explain Docker Compose

A tool for creating and executing multi-container Docker applications is called Docker Compose.

Definition: Code in a YAML file can be used to specify the containers for your application. Typically, the file is called docker-compose.yml.

Run Containers: All of the containers from your configuration file can be created and started with a single command.

Manage Containers: Commands can be used to start, stop, and rebuild services, among other aspects of your application’s lifecycle management.

Automate Tasks: Tasks like setting up a Docker network for your project and controlling Docker storage volumes are automated by Docker Compose.  

Benefits of Docker Compose:

The following are some advantages of Docker Compose:

• Streamlined development: You can make managing your whole application stack easier.
• Version control: You may easily distribute and allow others to contribute by keeping your application stack in a file located at the root of your project repository.
• Repeatable: The same setup will be used each time your containers run.
• Versatile: Docker Compose is compatible with development, testing, staging, production, and continuous integration workflows. 

23. What is a docker-compose.yml file?

The services, networks, and volumes for a multi-container application are specified in this YAML file. YML Services in a docker-compose.yml file stand in for the containers that the application will generate. 

A docker-compose.yml file with the services mentioned will be included when you create a new Divio application using one of our get started templates. 

24. How can you improve the security of your Docker images?

Use a secure supply chain, scan for vulnerabilities, update images frequently, and start with a minimum base image.

• Avoid root permissions.
• Secure Container Registries should be used.
• Reduce the Use of Resources.
• Examine your photos.
• Create secure networks and APIs.
• Docker Container Tracking.

25. What is Docker image signing?

The method of digitally signing Docker images is used to verify the program author’s identity and make sure the code hasn’t been changed or compromised.

• Digital signatures can be used for data transferred to and received from distant Docker registries due to Docker Content Trust (DCT).
  • This enables users to confirm the publisher and integrity of particular picture tags.
• The docker trust inspect command can be used by users to determine whether a Docker image is signed.
  • Which image tags are signed, who signed them, and who can sign new tags are among the JSON details on signed repositories that are provided by this command.  

Docker images are read-only templates that include container creation instructions. They serve as a snapshot of the dependencies and libraries needed for a program to function. 

26. What is Docker Swarm?

You can scale and manage a cluster of Docker nodes with Docker Swarm, a native clustering solution for Docker.

A container orchestration tool called Docker Swarm enables users to control a collection of Docker containers as a single virtual system.

With Docker Swarm, users can:

• Create a cluster by joining several real or virtual machines.
• Start the Docker containers.
• Attach containers to several hosts.
• Manage the resources of every node.
• Increase the availability of applications
• Assign a fair distribution of requests to services.
• Scale up or down automatically
• Restore environments to their former secure states.  

Several Docker instances can be combined into a single virtual host using Docker Swarm. The cluster’s operations are managed by a swarm manager. The cluster consists of:

• Nodes: Managers and employees are the two categories of nodes. Worker nodes receive and carry out tasks, while manager nodes oversee the cluster.
• Services: According to the declarative paradigm used by Docker Swarm, users specify the intended state of the service, and Docker keeps it up to date.
• Load Balancers: Docker Swarm provides load balancing automatically.

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27. What are the features of Docker Swarm?

Included in Docker Swarm are:

• Declarative service model: The different services in an application stack can have their desired states specified by the user.
• Reconciliation of the desired state: The swarm manager node keeps track of the cluster state and resolves any discrepancies between the desired and actual states.
• Decentralized access: Teams may easily access and administer the environment with Swarm.
• High security: There is very little risk of communication between the management and client nodes.  

28. Explain the concept of Docker layering.

Every instruction in the Dockerfile generates a new layer, which is how Docker images are constructed. This increases the efficiency of image construction.

Building Docker images by stacking several read-only layers on top of one another is known as Docker layering:

• Layers: A collection of file system modifications, like creating or editing a file, are represented by each layer. Dockerfile instructions build layers, and once they are formed, they cannot be changed.
• Caching: In order to reuse intermediate layers in later builds if they haven’t changed, Docker caches them. This saves resources and expedites builds.
• Order of instructions: Caching is impacted by the Dockerfile’s instruction sequence. In the Dockerfile, instructions that change often should be near the bottom, and those that change infrequently should be near the top.  
• Sharing: Any container that starts from the same image can share read-only layers. As a result, less local storage is needed.
• Manifest: A file that describes how to arrange the layers is called a manifest. It indicates which files should be downloaded and in what order they should be unpacked.
• Storage: The location of Docker images is /var/lib/docker/overlay2.  

The essential components for developing, deploying, and scaling systems are Docker layers. They function similarly to Git and aid in preventing the transfer of unnecessary data, Ignore the build steps that have not been modified, Determine whether the content is identical and whether the files were corrupted during the download.  

29. What is a Docker image ID?

It is a unique identifier for a Docker image. A SHA256 digest that identifies an image’s layers and configuration is called a Docker image ID.

What it is composed of: The SHA256 hash of the image’s JSON configuration object is contained in a digest called a Docker image ID. The SHA256 technique is used to calculate the layer digests, which are listed in the configuration object.

What it recognizes: Whereas a container ID identifies a container, a Docker image ID identifies an image.

How it’s applied: To retag an image, use the docker tag command. The docker rmi command can also be used to eliminate an image based on its digest.  

How a container ID differs from it: Although a container ID identifies a distinct type of item, it shares the same form as an image ID. 

How it’s different from a tag: Even if a picture has many tags, they will all share the same image ID. 

For instance, despite having distinct names, the images debian:bookworm and debian:latest share the same image ID. 

30. What is a Docker container ID?

An operating container can be uniquely identified by its Docker container ID:

How to get a container ID: You can obtain a container ID,

• By starting or constructing a new container and storing the output in a shell variable, 
• To obtain the IDs of every container that is currently running, use the docker ps command.

Container IDs are shown as follows: For ease of identification, the first 12 characters of a container ID are shown in the majority of Docker interfaces.

Using a container ID: Anywhere a container identifier is required, you can use it.

How to give a container a unique name: The –name parameter can be used to specify a unique container name.

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31. List some common use cases for Docker.

Docker has many common use cases, including: 

• Microservices architecture: The microservices architecture, which divides an application into separate services, is deployed using Docker.
• Continuous deployment: Docker makes it simple to accomplish continuous delivery and integration by facilitating the quick deployment of apps.
• Docker registry: In addition to controlling image distribution, Docker Registry enhances the security and access control of the images kept in its repository.
• DevOps: By unifying the configuration interface and streamlining machine setup, Docker streamlines DevOps.
• CI/CD pipelines: Applications may be efficiently tested, built, and deployed with Docker.  

Docker Real Time Interview Questions and Answers for Experienced

32. What is Docker Orchestration?

A collection of tools and procedures called Docker orchestration is used to manage Docker containers on a large scale. Provisioning, deployment, scaling, and administration of containers are all automated.

• Docker Swarm is an orchestration tool for containers, while Docker is a platform for developing containers. 
• Containerized applications can also be managed with container orchestration technologies such as Azure Kubernetes Service (AKS) and Google Kubernetes Engine (GKE). 

Organizations can benefit from container orchestration:

• Reduce maintenance overhead: Processes that would otherwise be laborious, error-prone, and manual are automated by container orchestration.
• Improve collaboration: Container orchestration improves cooperation between development and operations teams and connects with CI/CD workflows.
• Optimize DevOps productivity: Teams can do less system support and troubleshooting when they have capabilities like health monitoring and self-healing.
• Free up resources: When necessary, container orchestrators assign resources to containers; when not in use, they remove them.  

33. Explain the concept of Docker secrets.

One tool that enables users to safely store and manage sensitive data in Docker Swarm environments is called Docker secrets: A secure method for handling and storing private data in a Docker environment, such as API keys and passwords. 

Docker Secrets: Passwords, tokens, SSH keys, and other sensitive data can be stored in encrypted data blobs called Docker secrets.

How it works: Only service nodes with the vault key can access the vault where Docker secrets are kept. The secret is given to the service’s containers after being encrypted.  

How are Docker secrets used?: Docker Compose and Docker Swarm both support Docker secrets. Secrets in Docker Compose are referenced to a file in the working directory and declared in Compose files. The file gets mounted into the container when Docker Compose up is executed.

Advantages of Docker secrets

Docker secrets have various advantages, such as:

• Secure storage: Both in transit and at rest, secrets are encrypted.
• Managed lifecycle: Without restarting containers, secrets can be added, modified, and deleted.
• Access control: Secrets can only be retrieved by services that have been explicitly authorized access.
• Prevents inadvertent information exposure: When environment variables are used, secrets reduce the possibility of inadvertent information exposure.  

Only in swarm mode, which by default encrypts all communication between nodes, are Docker secrets accessible. 

34. What are Docker Labels?

Docker objects (images, containers) can have metadata added to them for simpler categorization and identification. One way to add metadata to Docker objects is using labels, which include:

• Images
• Containers
• Local daemons
• Volumes
• Networks
• Swarm nodes
• Swarm services

Labels can be used for any purpose that makes sense for your application or company, including organizing your photos, documenting license details, and annotating relationships across containers, volumes, and networks. 

35. How can a non-starting Docker container be troubleshooted?

If your Docker container isn’t beginning, consider these solutions:

• Examine the logs: Examine the container logs for any cautions, error messages, or other information.
• Examine the daemon for Docker: To find out if Docker is running, use the docker info command. Operating system tools such as sudo systemctl is-active docker are also available.
• Restart Docker Desktop: Issues are frequently fixed by restarting Docker Desktop.
• Clear out the mapped volumes: Stale data in these files may cause problems if your containers use mapped volumes.
• Docker resources should be pruned: Clear off unnecessary Docker resources on a regular basis to make disk space available.
• Restart your computer: Some issues can be resolved by restarting your system.
• Enhance Docker Desktop: Look for Docker Desktop for Windows updates.
• Reset Docker Desktop: Returns every setting to its starting point.
• Check the image of the container: Verify that the Docker image being used to build the container is accurate and current.

To list all containers, including ones that have stopped, you can also use the docker ps -a command. The output’s STATUS column will show if a container is stopped or operating. 

36. How may network connectivity problems with Docker containers be resolved?

When using Docker containers to troubleshoot network connectivity difficulties, you can:

• Verify the network setup: To verify the network setup, use the docker network inspect command.
• Make sure the right network is connected to the containers: Verify that the appropriate network is connected to the containers.
• Verify the network isolation policies and firewall settings: Verify that the network isolation policies and firewall settings are accurate.
• Make use of resources like traceroute and ping: To identify issues with connectivity, use tools like ping and traceroute.
• The docker network ls command should be used: To view a list of accessible networks, use the docker network ls command.
• Examine the logs from the container: Look through the container logs to find any cautions, error messages, or other details.  
• Check the image of the container: Make sure the Docker image is accurate and current before creating the container. 

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37. What are some best practices for writing Dockerfiles?

The following are some guidelines for creating Dockerfiles:

• Reduce the quantity of layers: Combining the RUN, COPY, and ADD commands results in fewer layers, which makes the image larger.
• Use COPY instead of ADD: As ADD can lead to harmful data or MITM attacks, use COPY instead. Additionally, ADD unpacks local archives implicitly, which may result in vulnerabilities related to Zip Slip and path traversal.
• Employ minimum base images: Although base images are practical, they may contain unnecessary components, which might cause security issues.
• Don’t use the most recent tag: Using the most recent tag may cause unknowns to enter the environment or break functionality.
• Put commands in the right order: Concatenate RUN instructions to reduce the number of layers and improve readability of the Dockerfile.  
• Use build cache: If all of the earlier steps are cached, then each build step caches the outcomes.
• Version control: Similar to source code, Dockerfiles can be versioned, which facilitates tracking changes, rolling back to earlier iterations, and maintaining various settings.
• Check for security flaws: Run as a user that is not root and secure the injected credentials.
• Managing multiline arguments: Understand the distinction between ENTRYPOINT and CMD.
• Use STDIN to build images without a Dockerfile.  

38. What are some best practices for securing Docker environments?

The following are recommended procedures for protecting Docker environments:

• Make use of safe images: Make use of authorized or validated photos from reliable sources, such as Docker Hub’s Verified Publisher. Steer clear of third-party public registers as they could not have control policies.
• Maintain Docker’s updates: Update the host operating system and Docker frequently to address security issues.
• Keep an eye on container activity: To find irregularities and look into dangers, track and record container activity.
• Make use of container security tools: Automate security policies, provide audit trails, and identify risks in real time with technologies like Sysdig, Twistlock, and Aqua Security.
• Decrease the default privileges: Limit the Docker container’s rights.
• Employ lean containers: Lean containers help you minimize your attack surface.
• Configure permissions: Make sure that the file system and volumes have read-only permissions.
• Picture scanning: Before using any container picture, make sure it is regularly scanned.
• Use Docker Content Trust: To confirm the legitimacy of an image, use Docker Content Trust.
• Implement network segmentation: Put network segmentation into practice.
• Protect sensitive information: Protect sensitive information.
• Dockerfile Linting: Lint your Dockerfiles while the build is running.
• Keep the Docker daemon socket hidden: Keep the Docker daemon socket hidden.
• Reduce the amount of resources used by the container: Reduce the amount of resources used by the container.
• Launch Docker without a root account: Launch Docker without a root account.  

39. How does Docker fit into a DevOps workflow?

A DevOps workflow can benefit from the use of Docker, a platform that assists developers in creating, deploying, and managing containers, in a number of ways.

• Create private containers: To generate private images and projects for testing within development, developers can utilize Docker.
• Package and execute the application code: Docker is a tool that developers may use to bundle and execute application code in lightweight containers that come with all the necessary components.
• Reduce deployment times: By using the same image for all deployments, Docker containers can shorten deployment times.
• Establish a stable atmosphere: Docker encapsulates apps into containers to give them a uniform and portable environment.
• Encourage DevOps and agile initiatives: Containers facilitate faster deployment, patching, and scaling of programs, supporting DevOps and agile initiatives.  

40. What advantages does Docker offer in a DevOps setting?

In a DevOps setting, Docker offers numerous advantages, such as:

• Deployment Productivity: Docker is a centralized program that enables users to coordinate multi-container applications, automate deployment, and generate containers.
• Security: To make sure that no container may access the processes operating within another container, Docker isolates and segregates resources and applications.
• Continuous deployment and testing: Continuous integration and deployment (CI/CD) techniques, which automate the software delivery process, are made possible by Docker.
• Environment standardization: Docker reduces inconsistencies between environments by offering clear guidelines for environment creation.  
• Control over changes: Users can make modifications and revert to earlier versions using Docker containers and images.
• Portability: Docker ensures that features that function well in one environment will also function well in others.
• Scalability: Docker containers can be quickly scaled to accommodate a workload that changes over time.
• Application topology: It consists of a number of interrelated components which is made easier to create with Docker.
• Load balancing: With Ingress and integrated service principles, Docker simplifies the load balancing configuration procedure.  

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Conclusion

We hope these top 40 interview questions on Docker along with answers help you review your skills on Docker containerization. Enroll in our Docker training in Chennai for a promising career in DevOps.