Top 50 Docker Interview Questions & Answers 2026

Quick Answer: This is the complete list of 50 Docker interview questions and answers for 2026 — images, Dockerfiles, volumes, networking, Compose, security, and orchestration. Each answer is short and interview-ready for DevOps and cloud roles.

Docker Fundamentals

1. What is Docker?
   An open-source platform to build, ship, and run applications in lightweight, portable containers.
2. What is a container?
   A lightweight, isolated process that packages an app with its dependencies and shares the host OS kernel.
3. What is the difference between a container and a virtual machine?
   Containers share the host kernel and are lightweight; VMs run a full guest OS and are heavier.
4. What is a Docker image?
   A read-only template containing the app and its dependencies, used to create containers.
5. What is the difference between an image and a container?
   An image is the static template; a container is a running instance of it.
6. What is Docker Engine?
   The core client-server application: the daemon (dockerd), REST API, and CLI.
7. What is a Docker registry?
   A store for images — public (Docker Hub) or private (GHCR, ECR, Harbor).

Images & Dockerfiles

8. What is a Dockerfile?
   A text file with instructions to build a Docker image.
9. Name common Dockerfile instructions.
   FROM, RUN, COPY, ADD, WORKDIR, ENV, EXPOSE, CMD, ENTRYPOINT.
10. What is the difference between CMD and ENTRYPOINT?
    ENTRYPOINT sets the fixed executable; CMD provides default, overridable arguments.
11. What is the difference between COPY and ADD?
    COPY just copies files; ADD also handles remote URLs and auto-extracts archives.
12. What are image layers?
    Each Dockerfile instruction creates a cached, read-only layer; containers add a writable top layer.
13. What is a multi-stage build?
    Using multiple FROM stages to keep build tools out of the final image for smaller, safer images.
14. How do you reduce image size?
    Minimal base images (Alpine/distroless), multi-stage builds, fewer layers, and a .dockerignore.
15. What is the build cache?
    Docker reuses unchanged layers to speed up rebuilds; ordering instructions well maximizes cache hits.

Containers & Lifecycle

16. How do you run a container in the background?
    docker run -d <image> (detached mode).
17. How do you list running vs all containers?
    docker ps for running; docker ps -a for all.
18. How do you enter a running container?
    docker exec -it <id> bash.
19. What is the difference between stop and kill?
    stop sends SIGTERM (graceful); kill sends SIGKILL (immediate).
20. How do you view container logs?
    docker logs <id>.
21. What is a restart policy?
    Controls auto-restart behavior: no, on-failure, always, unless-stopped.

Storage & Networking

22. How do you persist data in Docker?
    Use volumes (Docker-managed) or bind mounts (host paths).
23. What is the difference between a volume and a bind mount?
    Volumes are managed by Docker and portable; bind mounts map a specific host directory.
24. What is a tmpfs mount?
    Storage held in host memory only, never written to disk.
25. What are the Docker network drivers?
    bridge (default), host, none, overlay (multi-host), and macvlan.
26. What is the default network for containers?
    The bridge network.
27. How do containers on a user-defined bridge communicate?
    By container name via Docker’s built-in DNS.
28. What is port mapping?
    Exposing a container port to the host with -p host:container.

Docker Compose

29. What is Docker Compose?
    A tool to define and run multi-container apps from a YAML file (docker compose in v2).
30. What is the difference between docker run and Compose?
    run starts one container; Compose orchestrates multiple services declaratively.
31. What does depends_on do?
    Controls start order; combine with healthchecks to wait for readiness.
32. How do you scale a service in Compose?
    docker compose up --scale web=3.

Security & Best Practices

33. How do you secure Docker images?
    Use trusted minimal base images, scan for CVEs (docker scout/Trivy), run as non-root, and sign images (Cosign).
34. Why avoid running containers as root?
    It limits the blast radius if a container is compromised; use the USER instruction.
35. How do you manage secrets in Docker?
    Use Docker secrets, build secrets, or external secret managers — never bake secrets into images.
36. What is a .dockerignore file?
    Excludes files from the build context to speed builds and avoid leaking sensitive files.
37. What is image vulnerability scanning?
    Analyzing image layers for known CVEs before deployment.
38. How do you clean up unused Docker resources?
    docker system prune -a.

Orchestration & Advanced

39. What is Docker Swarm?
    Docker’s native clustering and orchestration tool.
40. What is the difference between Docker Swarm and Kubernetes?
    Swarm is simpler and Docker-native; Kubernetes is more powerful and the industry standard. See Docker Compose vs Kubernetes.
41. What is a container runtime?
    Software that runs containers; Docker uses containerd under the hood.
42. What is the OCI?
    The Open Container Initiative — standards for image and runtime formats.
43. What is a healthcheck?
    A command Docker runs to determine if a container is healthy.
44. What is BuildKit?
    Docker’s modern, faster build engine with caching and build secrets, default in current versions.
45. What is the difference between EXPOSE and -p?
    EXPOSE documents a port; -p actually publishes it to the host.
46. How do you debug a container that won’t start?
    Check docker logs, run interactively, inspect the entrypoint, and verify resource/port conflicts.
47. What is the difference between docker save and docker export?
    save exports an image (with layers); export exports a container’s filesystem.
48. How do you tag and push an image?
    docker tag img repo/img:tag then docker push repo/img:tag.
49. How do you set up a private registry?
    Run the registry image or use a managed one — see our local Docker repository guide.

Frequently Asked Questions

Are these Docker questions suitable for freshers?

Yes — fundamentals and Dockerfile sections suit freshers, while security, orchestration, and debugging target experienced candidates.

What are the most important Docker topics for interviews?

Images vs containers, Dockerfiles, multi-stage builds, volumes, networking, Compose, security, and how Docker relates to Kubernetes.

Is Docker still relevant in 2026?

Yes — Docker remains the standard for building and running containers and is foundational knowledge for Kubernetes and modern DevOps.

Related: Docker Tutorial · Kubernetes Interview Questions · DevOps Interview Questions