1. What is "Docker"?

Docker is an open-source platform that automates the deployment, scaling, and management of applications using containerization. It allows developers to package an application with all its dependencies and libraries, ensuring it runs consistently across different environments.

2. What is "Containerization"?

A lightweight form of virtualization that involves packaging an application and its dependencies into a single container that can run on any OS.

3. What's the difference between "Virtual Machines" (VMs) and "Containers"?

VM: Each has its own full OS; uses a hypervisor; heavier; slow to start.
Container: Shares the host OS kernel; uses Docker engine; much lighter; starts in seconds.

4. What is a "Docker Image"?

A read-only template that contains the instructions for creating a Docker container. Images are created using a Dockerfile.

5. What is a "Docker Container"?

A runnable instance of a Docker image. It is a lightweight, standalone, and executable package that includes everything needed to run an application.

6. What's the difference between an "Image" and a "Container"?

Image: A blueprint or recipe (static).
Container: A running instance of that blueprint (active).

7. What is a "Dockerfile"?

A text file that contains a collection of commands that Docker uses to build an image automatically.

8. What's the difference between `COPY` and `ADD` in a Dockerfile?

COPY: Simply copies files from the host to the container.
ADD: More powerful; can also download files from URLs and automatically extract tarballs.

9. What is the "Docker Hub"?

A cloud-based registry service where Docker users can store and share their container images.

10. What is a "Layered File System" in Docker?

Docker images are built using multiple layers. Each instruction in a Dockerfile creates a new layer, and layers are cached to speed up subsequent builds.

11. What is "Docker Compose"?

A tool for defining and running multi-container Docker applications. You use a YAML file (docker-compose.yml) to configure your application's services.

12. What is "Docker Swarm"?

Docker's native clustering and orchestration tool. It allows you to manage multiple Docker hosts as a single virtual host.

13. What is "Docker Engine"?

The core software that runs on the host machine and allows you to create, manage, and run Docker containers and images.

14. What's the difference between `RUN`, `CMD`, and `ENTRYPOINT`?

RUN: Executes a command during the image build process (creates a new layer).
CMD: Sets the default command to be executed when the container starts (can be overridden).
ENTRYPOINT: Sets the main command for the container; behaves like an executable (harder to override).

15. What are "Docker Volumes"?

Used for persisting data generated and used by Docker containers. Volumes are managed by Docker and stored on the host's file system, independent of the container's lifecycle.

16. What is "Bind Mounts"?

Similar to volumes, but they map a specific path on the host machine directly to a path in the container.

17. What is "Docker Network"?

A feature that allows containers to communicate with each other and with the outside world. Types include bridge (default), host, none, and overlay.

18. What is the "Docker Registry"?

A storage and distribution system for Docker images (Docker Hub is a public registry; you can also have private ones).

19. What is "Docker Exec"?

Use to run a command inside a currently running container (e.g., docker exec -it container_id /bin/bash).

20. What is "Docker Inspect"?

Provides detailed low-level information about a Docker object (image, container, network, volume) in JSON format.

21. What is the difference between `docker stop` and `docker kill`?

docker stop: Sends a SIGTERM signal, allowing the container to shut down gracefully.
docker kill: Sends a SIGKILL signal, stopping the container immediately (less safe).

22. What is "Orchestration" in the context of Docker?

The process of managing the lifecycle of containers, including deployment, scaling, load balancing, and networking (e.g., Kubernetes, Docker Swarm).

23. What is "Docker Prune"?

A command used to clean up unused Docker objects (stopped containers, unused networks, dangling images).

24. What is "Multi-stage Builds"?

A feature in Docker that allows you to use multiple FROM instructions in a single Dockerfile, creating a smaller final image by only copying necessary artifacts from previous stages.

25. What is the "Docker Daemon"?

A background process that runs on the host and manages Docker objects, listening for API requests from the Docker client.

26. What is "Docker Desktop"?

An easy-to-install application for Mac and Windows that provides a GUI and all the necessary tools to build and share containerized applications.

27. What is the `docker build` command?

Builds a Docker image from a Dockerfile and a context (the directory containing the file).

28. What is the `docker ps` command?

Lists all currently running Docker containers. (Add -a to see stopped containers as well).

29. What is the `docker images` command?

Lists all locally stored Docker images.

30. What is "Contianer Lifecycle"?

The stages a container goes through: Created, Running, Paused, Stopped, Deleted.

31. What is "Docker Registry" vs "Docker Repository"?

Registry: The service that stores images (e.g., Docker Hub).
Repository: A collection of related images with the same name but different tags (e.g., ubuntu:20.04, ubuntu:latest).

32. What is "Dangling Image" in Docker?

A layer that is no longer associated with any tagged image, often created during the build process if a new image version is created with the same tag.

33. What is "Instruction Cache" in Docker?

The mechanism where Docker reuse layers from previous builds if the instructions in the Dockerfile haven't changed, significantly speeding up the build process.

34. What is "Docker Logs"?

Provides access to the output (stdout/stderr) of a running or stopped container.

35. What is "Docker Port Mapping"?

Connecting a port on the host machine to a port inside the container so that services can be accessed from outside the host (e.g., -p 8080:80).

36. What is "Docker Entrypoint" vs "Docker Command"?

Entrypoint: The main process being run.
Command: Parameters passed to the Entrypoint.

37. What's the main purpose of using "Docker in CI/CD"?

To ensure that the application is built, tested, and deployed in the exact same environment at every step of the pipeline, reducing "it works on my machine" issues.

38. What is "Docker Content Trust"?

A feature that allows you to sign and verify Docker images to ensure they haven't been tampered with.

39. What is "Secret management" in Docker?

A way to securely store and share sensitive information (like passwords or API keys) with containers without baking them into the image.

40. What is "LCOW" (Linux Containers on Windows)?

A technology that allows Linux containers to run natively on Windows using a lightweight Linux kernel and the Windows Hyper-V.

41. What is the purpose of `EXPOSE` in a Dockerfile?

It serves as documentation, informing the user about the ports the container listens on at runtime. It doesn't actually open the ports on the host.

42. What is the `WORKDIR` instruction?

Sets the working directory for any subsequent RUN, CMD, ENTRYPOINT, COPY, or ADD instructions in the Dockerfile.

43. What is the difference between `docker rm` and `docker rmi`?

docker rm: Removes a container.
docker rmi: Removes an image.

44. What is the "Namespace" and "Cgroups" in Docker?

Linux kernel features that Docker uses for isolation:

Namespace: Isolates process resources (IDs, network, files).
Cgroups: Limits and monitors resource usage (CPU, memory).

45. What is the `ENV` instruction?

Sets environment variables that are available both during the build process and inside the running container.

46. What is the `ARG` instruction?

Defines a variable that users can pass to the builder at build-time using docker build --build-arg <varname>=<value>. It's not available in the running container.

47. What is "Kubernetes" and how does it relate to Docker?

Kubernetes is a powerful orchestration platform that manages clusters of Docker containers across multiple hosts.

48. What is the "Docker Scan" command?

A tool used to scan images for known security vulnerabilities.

49. What is the `docker login` command?

Authenticates the Docker client with a Docker registry (like Docker Hub) so you can push and pull private images.

50. Why is Docker so popular among developers?

Because it provides lightweight isolation, reproducibility, easy dependency management, and a huge library of pre-built images.

Docker Interview Questions

Table of Contents

What is "Docker"?

What is "Containerization"?

What's the difference between "Virtual Machines" (VMs) and "Containers"?

What is a "Docker Image"?

What is a "Docker Container"?

What's the difference between an "Image" and a "Container"?

What is a "Dockerfile"?

What's the difference between <code>COPY</code> and <code>ADD</code> in a Dockerfile?

What is the "Docker Hub"?

What is a "Layered File System" in Docker?

What is "Docker Compose"?

What is "Docker Swarm"?

What is "Docker Engine"?

What's the difference between <code>RUN</code>, <code>CMD</code>, and <code>ENTRYPOINT</code>?

What are "Docker Volumes"?

What is "Bind Mounts"?

What is "Docker Network"?

What is the "Docker Registry"?

What is "Docker Exec"?

What is "Docker Inspect"?

What is the difference between <code>docker stop</code> and <code>docker kill</code>?

What is "Orchestration" in the context of Docker?

What is "Docker Prune"?

What is "Multi-stage Builds"?

What is the "Docker Daemon"?

What is "Docker Desktop"?

What is the <code>docker build</code> command?

What is the <code>docker ps</code> command?

What is the <code>docker images</code> command?

What is "Contianer Lifecycle"?

What is "Docker Registry" vs "Docker Repository"?

What is "Dangling Image" in Docker?

What is "Instruction Cache" in Docker?

What is "Docker Logs"?

What is "Docker Port Mapping"?

What is "Docker Entrypoint" vs "Docker Command"?

What's the main purpose of using "Docker in CI/CD"?

What is "Docker Content Trust"?

What is "Secret management" in Docker?

What is "LCOW" (Linux Containers on Windows)?

What is the purpose of <code>EXPOSE</code> in a Dockerfile?

What is the <code>WORKDIR</code> instruction?

What is the difference between <code>docker rm</code> and <code>docker rmi</code>?

What is the "Namespace" and "Cgroups" in Docker?

What is the <code>ENV</code> instruction?

What is the <code>ARG</code> instruction?

What is "Kubernetes" and how does it relate to Docker?

What is the "Docker Scan" command?

What is the <code>docker login</code> command?

Why is Docker so popular among developers?