Docker Interview Questions

Finally!! 🎉

You have completed ✅ the Docker hands-on sessions, and I hope you have learned something valuable from it. 🙌

Docker being a crucial topic for DevOps Engineer interviews, especially for freshers. Here are some essential questions to help you prepare and ace your Docker interviews:

1. Difference between an Image, Container, and Engine:

   • Image: A read-only template with instructions to create a Docker container.

   • Container: A running instance of a Docker image.

   • Engine: The runtime that manages containers on a host system.

2. Difference between the Docker command COPY vs ADD:

   • COPY: Copies files/directories to the container without any additional functionality.

   • ADD: Similar to COPY but also supports fetching remote URLs and extracting archives.

3. Difference between the Docker command CMD vs RUN:

   • CMD: Sets the default command to run when the container starts.

   • RUN: Executes a command during the build process and creates a layer in the image.

4. How will you reduce the size of a Docker image?

   • Use multi-stage builds, minimize the number of layers, remove unnecessary files, and use .dockerignore.

5. Why and when to use Docker?

   • Use Docker for consistent environments, easy deployment, scalability, and isolation of applications for their easy shipment.

6. Explain the Docker components and how they interact with each other.

   • Components: Docker Engine, Docker Images, Docker Containers, Docker Daemon, Docker CLI, and Docker Compose.

   • Interaction: The Engine runs containers from images, managed by the Daemon, controlled via the CLI, and orchestrated by Compose.

7. Explain the terminology: Docker Compose, Dockerfile, Docker Image, Docker Container.

   • Docker Compose: Tool for defining and running multi-container Docker applications.

   • Dockerfile: Script with instructions to build a Docker image.

   • Docker Image: A template for creating Docker containers.

   • Docker Container: An isolated environment to run applications.

8. In what real scenarios have you used Docker?

   • Deploying microservices, CI/CD pipelines, isolating development environments, and running legacy applications.

9. Docker vs Hypervisor?

   • Docker: Lightweight containers sharing the host OS kernel.

   • Hypervisor: Virtual machines with full OS, more resource-intensive.

10. Advantages and disadvantages of Docker?

    • Advantages: Portability, efficiency, scalability, and faster deployment.

    • Disadvantages: Security concerns, networking complexity, and data persistence issues.

11. What is a Docker namespace?

    • A feature for isolating resources (e.g., PID, network) in containers.

12. What is a Docker registry?

    • A storage and distribution system for Docker images (e.g., Docker Hub).

13. What is an entry point?

    • The command that runs as the main process inside a container.

14. How to implement CI/CD in Docker?

    • Use Docker to build, test, and deploy containers in automated CI/CD pipelines.

15. Will data on the container be lost when the Docker container exits?

    • Yes, data is lost when a container exits unless volumes or bind mounts are used to backup the container data.

16. What is a Docker swarm?

    • A native clustering and orchestration tool for Docker containers.

17. What are docker commands for the following:

    • Viewing running containers: docker ps

    • Running a container under a specific name: docker run --name <name> <image>

    • Exporting a Docker image: docker save -o <file> <image>

    • Importing an existing Docker image: docker load -i <file>

    • Deleting a container: docker rm <container_id>

    • Removing all stopped containers, unused networks, build caches, and dangling images: docker system prune

18. What are the common Docker practices to reduce the size of Docker images?

    • Multi-stage builds, using smaller base images, minimizing layers, and cleaning up after package installations.

19. How do you troubleshoot a Docker container that is not starting?

    • Check logs with docker logs <container_id>, inspect with docker inspect, and verify configuration files.

20. Docker networking model:

    • Docker uses bridge, overlay, host, and macvlan networks to connect containers.

21. How do you manage persistent storage in Docker?

    • Use Docker volumes or bind mounts to persist data outside the container lifecycle.

22. How do you secure a Docker container?

    • Limit privileges, use secure base images, scan images for vulnerabilities, and use Docker security features like seccomp and AppArmor.

23. What is docker overlay networking:

    • A network that allows containers across different hosts to communicate securely in a Docker Swarm.

24. How do you handle environment variables in Docker?

    • Pass environment variables using -e or --env-file options when running containers.

I hope these questions will surely help you ace your next interview for Docker. Happy learning! 😊