1. What is Docker, and how is it different from a virtual machine?

Answer: Docker is a platform for developing, shipping, and running applications in containers. Unlike virtual machines (which virtualize hardware), Docker virtualizes the OS, making it lightweight and faster to start.

Feature	Virtual Machine	Docker
Virtualizes	Hardware	OS level
Startup Time	Minutes	Seconds
Resource Use	Heavy	Lightweight

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2. What is a Docker image?

Answer: A Docker image is a read-only template containing the application code, libraries, dependencies, and instructions needed to create a container.

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3. How do you create a Docker image?

Answer: By writing a Dockerfile and running docker build.

Example:

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Run:

docker build -t my-node-app .

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4. What is a Docker container?

Answer: A Docker container is a runnable instance of a Docker image. It includes the app and its environment, isolated from the host.

Run a container:

docker run -d -p 3000:3000 my-node-app

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5. Explain the difference between CMD and ENTRYPOINT in Docker.

Answer: Both define the command to run inside a container, but ENTRYPOINT is not overridden by command-line arguments, while CMD is.

Example:

ENTRYPOINT ["echo"]
CMD ["Hello"]

Running:

docker run myimage World

Output: echo World

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6. How do you view running Docker containers?

docker ps

To see all containers:

docker ps -a

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7. How do you stop a running container?

docker stop <container_id>

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8. What is the difference between docker stop and docker kill?

Answer: docker stop sends a SIGTERM and allows graceful shutdown; docker kill sends SIGKILL and forces immediate stop.

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9. How do you remove a Docker container?

docker rm <container_id>

Remove all stopped containers:

docker container prune

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10. How do you remove a Docker image?

docker rmi <image_id>

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11. What is Docker Hub?

Answer: Docker Hub is a cloud-based repository where Docker users can share, store, and distribute images.

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12. Explain Docker volumes and their use cases.

Answer: Volumes are Docker-managed directories stored on the host to persist data outside containers.

Mount a volume:

docker run -v my_volume:/data busybox

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13. Difference between bind mounts and volumes?

Feature	Bind Mounts	Volumes
Managed by	User	Docker
Path specified	Absolute path	Named or anonymous
Backups	Manual	docker volume command

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14. What is the Dockerfile instruction order significance?

Answer: Docker builds layers in the order of instructions. Changing early layers causes rebuilding of all subsequent layers.

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15. What is a multi-stage build in Docker? Why use it?

Answer: It allows using multiple FROM instructions to reduce final image size by copying only needed artifacts to the final stage.

Example:

FROM golang:1.18 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

FROM alpine
COPY --from=builder /app/myapp /myapp
CMD ["/myapp"]

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16. Explain Docker networking types.

Network Type	Use case
bridge	Default, single-host
host	Shares host network
overlay	Multi-host swarm
none	No networking

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17. How do you connect a container to a network?

docker network create mynet
docker run --network=mynet myimage

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18. What is a Docker context?

Answer: A context lets you switch between different Docker environments (e.g., local, remote servers).

List:

docker context ls

Use:

docker context use mycontext

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

COPY	ADD
Copies files/directories	Copies + supports URL, tar extraction

Prefer COPY unless you need extra ADD features.

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20. How do you pass environment variables to containers?

docker run -e VAR_NAME=value myimage

Or in Dockerfile:

ENV VAR_NAME value

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21. Explain docker exec vs docker attach.

Answer:

• docker exec: runs a command in a running container.

• docker attach: connects your terminal to container's main process.

Use exec for isolated command; attach to interact directly.

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22. What is a dangling image?

Answer: An image not tagged or referenced by any container, usually intermediate build layers.

Clean:

docker image prune

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23. What is Docker Swarm?

Answer: Native Docker clustering tool for managing multiple Docker hosts as a single virtual system.

Initialize:

docker swarm init

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24. How do you scale services in Docker Swarm?

docker service scale myservice=5

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25. How do you inspect a Docker container?

docker inspect <container_id>

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26. How do you limit CPU and memory usage in a container?

docker run --memory="500m" --cpus="1.5" myimage

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27. What is .dockerignore file?

Answer: Works like .gitignore to exclude files from the build context.

Example:

node_modules
*.log

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28. Explain Docker Compose.

Answer: A tool to define and run multi-container Docker apps using docker-compose.yml.

Example docker-compose.yml:

version: '3'
services:
  web:
    image: nginx
    ports:
      - "8080:80"
  db:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD: example

Run:

docker-compose up -d

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29. What is the difference between docker-compose up and docker-compose up --build?

Answer: up --build forces rebuilding images before starting containers.

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30. How do you persist database data in Docker?

By using a named volume:

docker run -v db_data:/var/lib/mysql mysql

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31. What is the difference between docker save and docker export?

Answer:

• docker save: saves an image (with layers and metadata) to a tar archive.

• docker export: exports a container's filesystem as a tar archive without metadata.

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32. How do you import/export Docker images?

Export:

docker save myimage > myimage.tar

Import:

docker load < myimage.tar

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33. How do you copy files from a running container to the host?

docker cp <container_id>:/path/in/container /host/path

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34. What is the difference between a container restart policy always vs unless-stopped?

Policy	Behavior
always	Always restarts
unless-stopped	Restart unless explicitly stopped

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35. What is the difference between image layers and containers?

Answer: Images are composed of immutable layers; containers are runnable instances that add a writable layer on top of the image layers.

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36. How do you clean up unused Docker resources?

docker system prune

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37. Explain what happens when you run docker run nginx.

Answer:

1. Docker checks for nginx image locally.

2. If not found, pulls from registry.

3. Creates container from image.

4. Starts container with default command.

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38. How do you configure logging drivers in Docker?

docker run --log-driver=json-file myimage

Available drivers: json-file, syslog, journald, gelf, fluentd, awslogs.

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39. How do you view Docker container logs?

docker logs <container_id>

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40. How do you debug a Docker build failure?

docker build --progress=plain --no-cache .

Also use docker history <image> to inspect layers.

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41. Explain Docker health checks.

Answer: Used to determine container health status.

In Dockerfile:

HEALTHCHECK CMD curl --fail http://localhost:8080 || exit 1

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42. What is a sidecar container?

Answer: A container that runs alongside a main application container, providing auxiliary functionality (logging, monitoring, proxy).

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43. How do you attach to a running container's shell?

docker exec -it <container_id> /bin/bash

If bash missing, try /bin/sh.

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44. How do you update a running container?

You can't directly update; you need to:

1. Commit changes:

docker commit <container_id> myupdatedimage

2. Run new container from updated image.

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45. What is Docker's default storage driver?

Depends on OS:

• overlay2 on most Linux distros.

• windowsfilter on Windows.

Check:

docker info | grep Storage

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46. How do you inspect an image's layers?

docker history <image>

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47. Explain container exit codes.

• 0: successful execution

• Non-zero: error code from process

Check exit code:

docker inspect --format='{{.State.ExitCode}}' <container>

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48. What is the difference between docker-compose down and docker-compose stop?

Command	Effect
stop	Stops containers
down	Stops + removes containers, networks, volumes

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49. Explain image caching in Docker builds.

Docker caches intermediate layers to speed up rebuilds; changes to earlier layers invalidate cache for later ones.

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50. How do you override an image's CMD at runtime?

docker run myimage mycustomcommand