Docker Interview Questions and Answers for 5 years experience

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100 Docker Interview Questions and Answers

  1. What is Docker?

    • Answer: Docker is an open-source platform to develop, ship, and run applications using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package. This guarantees that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.

  2. Explain the difference between a Docker image and a Docker container.

    • Answer: A Docker image is a read-only template with instructions for creating a Docker container. It's essentially a snapshot of the application and its dependencies. A Docker container, on the other hand, is a running instance of a Docker image. You can think of the image as the blueprint and the container as the house built from that blueprint.

  3. What is a Dockerfile?

    • Answer: A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build on this file, produces a ready to run image.

  4. Explain the concept of Docker layers.

    • Answer: Docker images are built layer by layer. Each instruction in a Dockerfile creates a new layer. This layered approach is efficient because changes only require rebuilding the affected layers, saving time and resources. It also enables caching of layers, further speeding up the image building process.

  5. What are Docker volumes?

    • Answer: Docker volumes provide a mechanism to persist data generated by and used by Docker containers. They are separate from the container's file system and are managed by Docker. This ensures data survives even if the container is deleted or migrated.

  6. How do you manage Docker networks?

    • Answer: Docker networks allow containers to communicate with each other. You can create different types of networks (bridge, host, overlay, macvlan) depending on your needs. The `docker network` command is used to manage these networks, creating, connecting, and inspecting them.

  7. Explain Docker Compose.

    • Answer: Docker Compose is a tool for defining and running multi-container Docker applications. It uses a YAML file (docker-compose.yml) to define the services, networks, and volumes needed for the application. This simplifies the management of complex applications with multiple containers.

  8. What is Docker Swarm?

    • Answer: Docker Swarm is a native clustering tool for Docker that allows you to manage a cluster of Docker nodes. It enables you to scale your applications across multiple hosts, providing high availability and increased capacity.

  9. What are Docker registries?

    • Answer: Docker registries are repositories for storing and managing Docker images. Docker Hub is a public registry, but you can also create your own private registries for storing your internal images.

  10. How do you build a Docker image from a Dockerfile?

    • Answer: The command `docker build -t : .` builds an image. The `-t` flag specifies the image name and tag, and the `.` indicates the current directory containing the Dockerfile.

  11. How to troubleshoot a container that's not running?

    • Answer: Check the container logs using `docker logs `. Inspect the container status using `docker inspect `. Examine the Dockerfile for errors. Check the host machine's resources (CPU, memory, disk space).

  12. Explain the concept of Docker security best practices.

    • Answer: Use minimal base images, regularly update images, scan images for vulnerabilities, restrict access to the Docker daemon, use non-root users inside containers, and employ secrets management.

  13. What are some common Docker commands you use daily?

    • Answer: `docker run`, `docker ps`, `docker stop`, `docker rm`, `docker build`, `docker images`, `docker exec`, `docker logs`, `docker inspect`.

  14. How do you handle persistent storage in Docker?

    • Answer: Use Docker volumes for persistent data. Volumes are managed by Docker and survive container restarts and removals.

  15. Explain the difference between `docker-compose up` and `docker-compose up -d`?

    • Answer: `docker-compose up` runs the containers in the foreground, while `docker-compose up -d` runs them in detached mode (background).

  16. How do you scale a service in Docker Swarm?

    • Answer: Use the `docker service scale` command to specify the desired number of replicas for a service.

  17. What are some alternatives to Docker?

    • Answer: Podman, containerd, rkt (Rocket), Kubernetes (while not a direct replacement, it offers similar container orchestration functionalities).

  18. Describe your experience with Docker in a production environment.

    • Answer: (This requires a personalized answer based on your experience. Mention specific projects, challenges faced, and solutions implemented using Docker in a production setting.)

  19. How do you optimize Docker image size?

    • Answer: Use multi-stage builds to reduce image size, use smaller base images, remove unnecessary files and packages, and leverage image compression techniques.

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