Top 30 Docker Interview Questions and Answers for All Experience Levels

docker create creates a container but doesn't start it, while docker run creates and starts the container immediately.[2]

8. What is a Dockerfile?

A Dockerfile is a text file containing instructions to build a Docker image. It defines the environment layer by layer using commands like FROM, RUN, COPY, and CMD.[1]

9. What command starts a stopped Docker container?

Use docker start <container_id> to start a stopped container.[4]

10. What does the -d flag do in docker run?

The -d flag runs the container in detached mode, allowing it to run in the background without attaching to the terminal.[4]

Intermediate Docker Interview Questions (11-20)

11. What is the difference between CMD and ENTRYPOINT in a Dockerfile?

CMD provides defaults for an executing container, while ENTRYPOINT configures the container as an executable. ENTRYPOINT cannot be overridden by arguments, but CMD can.[2]

12. What is the difference between COPY and ADD in a Dockerfile?

COPY is used to copy files from the host to the container, while ADD can also handle URLs and automatically extract tar archives.[2]

13. What are Docker volumes and why are they used?

Docker volumes provide persistent storage for containers. Data in volumes survives container deletion, unlike container filesystem data.[1]

14. What is the difference between "EXPOSE" and "publish" in Docker?

EXPOSE documents the port the container listens on in the Dockerfile, while -p or --publish actually maps that port to the host.[2]

15. How do you stop a running Docker container?

Use docker stop <container_id> to gracefully stop a container. It sends a SIGTERM signal and waits for cleanup.[4]

16. What command kills a Docker container immediately?

docker kill <container_id> sends a SIGKILL signal to immediately terminate the container without cleanup.[4]

17. What is a Docker registry?

A Docker registry is a storage and distribution system for Docker images. Public registries like Docker Hub host public images, while private registries store proprietary ones.[3]

18. How do you access a shell inside a running Docker container?

Use docker exec -it <container_id> /bin/bash to get an interactive shell in a running container.[6]

19. What happens to data when a Docker container exits?

Data stored in the container's writable layer is lost when the container is deleted, unless persisted using volumes or bind mounts.[5]

20. In a Flipkart deployment scenario, how would you run a container with limited memory?

Use docker run -m 512m <image> to limit container memory to 512MB, preventing it from consuming excessive host resources.[2]

Advanced Docker Interview Questions (21-30)

21. What is Docker Compose and what file does it use?

Docker Compose is a tool for defining and running multi-container Docker applications using a YAML file called docker-compose.yml.[6]

22. Explain Docker build cache and how it works.

Docker build cache speeds up image builds by reusing layers from previous builds when Dockerfile instructions haven't changed.[2]

23. What is an orphan volume and how do you remove it?

An orphan volume is a volume not referenced by any container. Remove it with docker volume prune after confirmation.[2]

24. How does Docker handle networking between containers?

Docker provides default bridge networking. Containers can communicate using container names as DNS hostnames on the same network.[1]

25. What is the ONBUILD instruction in Dockerfile?

ONBUILD sets instructions to execute when the image is used as a base for another image, useful for creating reusable base images.[2]

26. In a Paytm microservices scenario, how would you control service startup order in Docker Compose?

Use depends_on in docker-compose.yml to define startup dependencies between services, ensuring databases start before applications.[2]

27. What command removes all stopped containers, unused networks, and dangling images?

docker system prune removes stopped containers, unused networks, build caches, and dangling images to free up space.[3]

28. How do you transfer a Docker image between machines without a registry?

Save the image with docker save -o image.tar <image>, transfer the tar file, then load with docker load -i image.tar.[2]

29. For a Zoho SaaS application, how would you make Docker Compose production-ready?

Remove volume bindings, add restart policies, configure log aggregation services, and bind to specific host ports in docker-compose.yml.[4]

30. What is the default CPU limit for a Docker container?

Docker containers have no default CPU limit; they can use all available host CPU unless limited using --cpus or --cpu-shares flags.[2]

# Example production-ready Dockerfile for a web app
FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
EXPOSE 3000
USER node
CMD ["npm", "start"]

Master these Docker interview questions to showcase your skills from basic container management to advanced orchestration. Practice hands-on with Docker commands and Dockerfiles to confidently handle any interview scenario.