🚀 Day 21 of My #90DaysOfDevOps Challenge: Important Docker Interview Questions 🚀

Docker is an important tool for DevOps engineers, especially for freshers. Today, I am sharing some common Docker interview questions and simple answers to help you understand and prepare better. Let’s get started! 😊

🔹 Docker Interview Questions and Answers

1. What is the difference between an Image, Container, and Engine?

Image: A read-only template that has everything needed to run an application (code, libraries, etc.). Think of it like a recipe.
Container: A running version of the image. It’s like the dish you make using the recipe.
Engine: The engine (Docker Engine) is the software that makes Docker work. It creates and manages containers.

2. What is the difference between COPY and ADD in Docker?

COPY: Copies files or folders from your computer into the Docker image.
ADD: Does the same as COPY but can also download files from the internet or unzip files during the process.
Use COPY unless you need the extra features of ADD.

3. What is the difference between CMD and RUN in Docker?

RUN: Executes commands while building the Docker image. For example, installing software inside the image.
CMD: Specifies the default command to run when the container starts. For example, starting the app.
Tip: Use RUN for setup steps and CMD to define how the container should behave.

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

Use a smaller base image like alpine.
Avoid copying unnecessary files into the image.
Combine multiple RUN commands into one to reduce layers.

5. Why and when should you use Docker?

Why: Docker makes it easy to run and share applications in any environment without compatibility issues.
When: Use Docker when you need portability, scalability, or isolated environments for apps.

6. What are Docker components, and how do they work together?

Docker Engine: Runs and manages everything.
Docker Images: Templates that containers are based on.
Docker Containers: The running apps based on images.
Docker Registry: A storage place for Docker images (like Docker Hub).
They work together to build, store, and run apps smoothly.

7. Explain Docker Compose, Dockerfile, Docker Image, and Docker Container.

Docker Compose: A tool to run multiple containers together with one configuration file.
Dockerfile: A text file with steps to build a Docker image.
Docker Image: The blueprint (template) for creating containers.
Docker Container: A running version of an image, like an app in action.

8. Can you give real-world scenarios for using Docker?

Running a web app locally in the same way it will run on the server.
Isolating an application to avoid conflicts with other apps.
Testing a new tool or software without installing it on your system.

9. Docker vs. Hypervisor?

Docker: Lightweight, uses the host OS, and runs faster.
Hypervisor: Creates virtual machines with their own OS but uses more resources and runs slower.

10. What are the advantages and disadvantages of using Docker?

Advantages:
- Portable and works on any machine.
- Fast and uses fewer resources.
- Easy to scale apps.
Disadvantages:
- Not ideal for apps needing a full OS.
- Needs experience to secure containers properly.

11. What is a Docker namespace?

Namespaces isolate containers from each other, so each container thinks it’s running on its own system.

12. What is a Docker registry?

A storage place for Docker images. Examples: Docker Hub (public) or private registries for organizations.

13. What is an entry point in Docker?

It’s the command that automatically runs when a container starts.

14. How can Docker be used in CI/CD?

Use Docker to create identical environments for testing and deployment.
Integrate it with tools like Jenkins to automate testing and deployment.

15. Will data be lost when a container exits?

Yes, unless you use volumes to store the data outside the container.

16. What is a Docker swarm?

A tool to manage a group of Docker containers running on multiple machines.

17. Common Docker commands and their uses:

View running containers: docker ps
Run a container with a specific name: docker run --name <name> <image>
Export a Docker image: docker save -o <file_name> <image>
Import an image: docker load -i <file_name>
Delete a container: docker rm <container_name>
Clean unused items: docker system prune

18. How can you reduce Docker image size?

Use lightweight base images (like alpine).
Remove temporary files during the build process.
Combine related RUN commands to reduce layers.

19. How do you troubleshoot a Docker container that won’t start?

Check logs: docker logs <container_name>.
Inspect configuration in the Dockerfile.
Look for errors in the entry point or environment variables.

20. Can you explain Docker networking?

Docker creates networks so containers can talk to each other.
Default networks:
- bridge: For isolated containers.
- host: Shares the host’s network.
- none: No network.

21. How do you manage persistent storage in Docker?

Use volumes to store data that stays even when containers are deleted.

22. How do you secure a Docker container?

Use the least privileges required.
Scan images for vulnerabilities.
Avoid running containers as the root user.

23. What is Docker overlay networking?

It allows containers on different machines to communicate, like in Docker Swarm.

24. How do you handle environment variables in Docker?

Use the --env flag when running a container:

  docker run --env VAR_NAME=value <image>

Or define them in a Docker Compose file.

Conclusion

These Docker questions and answers will help you build confidence for interviews. Docker is a key tool in DevOps, so understanding it will make you stand out.

Keep learning, practicing, and sharing your knowledge with the DevOps community! 💪

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