Docker Interview Questions

Here are some advanced Docker interview questions, including topics on Docker Compose, Swarm, Docker Trusted Registry (DTR), and Docker Hub.

Docker Advanced Interview Questions

1. What are Docker namespaces and cgroups, and how do they work together?
   • Explain how namespaces isolate processes and cgroups manage resource allocation in containers.
2. What are Docker layers, and how does the layering system optimize image creation?
   • Describe the concept of layers in Docker images and how they reduce storage and increase efficiency.
3. What is the difference between a COPY and an ADD command in a Dockerfile?
   • Explain the differences in behavior, particularly how ADD handles URLs and archive extraction.
4. How does Docker ensure security for containers?
   • Discuss the use of AppArmor, SELinux, cgroups, namespaces, and other security measures.
5. How do multi-stage builds work in Docker, and why are they useful?
   • Describe how multi-stage builds reduce image size and improve build efficiency.
6. What are Docker volumes, and how do they differ from bind mounts?
   • Explain the differences between volumes and bind mounts, and discuss the pros and cons of each.
7. How do you troubleshoot performance issues in a Docker container?
   • Describe the steps for identifying performance bottlenecks using Docker’s built-in tools (e.g., docker stats, docker logs).

Docker Compose Interview Questions

8. What is Docker Compose, and what are its main use cases?
   • Explain how Docker Compose is used for defining and running multi-container applications.
9. What is the structure of a docker-compose.yml file?
   • Walk through the key sections, such as services, networks, and volumes.
10. How do you scale services in Docker Compose?
    • Describe how the docker-compose up --scale command works for horizontal scaling.
11. How does Docker Compose handle networking between containers?
    • Explain how Docker Compose creates a default network for services and how you can define custom networks.
12. What is the difference between depends_on, links, and network_mode in a Compose file?
    • Discuss how these configurations manage the order and connectivity of containers.
13. Can you use environment variables in a Docker Compose file? How?
    • Describe how to inject environment variables using the environment key, .env files, and docker-compose commands.
14. How do you implement persistent storage using Docker Compose?
    • Explain the use of the volumes key to persist data across container restarts.

Docker Swarm Interview Questions

15. What is Docker Swarm, and how does it work?
    • Provide an overview of Docker Swarm for orchestrating services across multiple nodes.
16. How do you initialize and manage a Docker Swarm cluster?
    • Describe the process of creating a Swarm (docker swarm init), adding nodes, and managing the cluster.
17. What are the different types of nodes in Docker Swarm?
    • Explain the roles of manager nodes and worker nodes in the Swarm architecture.
18. How do you deploy services in Docker Swarm?
    • Walk through the process of deploying services using docker service create and docker stack deploy.
19. How does Docker Swarm handle load balancing?
    • Discuss how Docker Swarm provides built-in load balancing between container replicas.
20. What are Docker Swarm secrets, and how do you use them?
    • Explain how to manage sensitive data (e.g., passwords, certificates) using Swarm secrets.
21. What is the difference between a replicated and global service in Docker Swarm?
    • Describe the scenarios for using replicated services versus global services.
22. How does Docker Swarm handle failover and high availability?
    • Discuss how Swarm automatically reschedules tasks when a node goes down and how it ensures high availability.

Docker Trusted Registry (DTR) Interview Questions

23. What is Docker Trusted Registry (DTR)?
    • Explain how DTR is an enterprise-grade Docker registry for storing and managing Docker images.
24. What is the difference between DTR and Docker Hub?
    • Discuss the enterprise features of DTR, such as security scanning and access control, which are not available in Docker Hub.
25. How does image signing work in Docker Trusted Registry?
    • Describe how Docker Content Trust and Notary are used to sign and verify Docker images in DTR.
26. How do you set up access control in Docker Trusted Registry?
    • Explain how DTR integrates with LDAP or Active Directory to enforce user-based access control.
27. What are the security features of DTR?
    • Discuss features like role-based access control (RBAC), image scanning for vulnerabilities, and audit logs.
28. How do you replicate images across multiple DTR instances?
    • Describe the image replication feature that allows images to be mirrored across multiple DTR instances for high availability.
29. What is a repository mirroring in DTR, and why is it used?
    • Explain how repository mirroring helps replicate or synchronize image repositories from other registries.

Docker Hub Interview Questions

30. What is Docker Hub, and what are its primary features?
    • Discuss the cloud-based registry service where Docker images can be stored, shared, and accessed.
31. How do you automate Docker image builds on Docker Hub?
    • Describe the process of setting up automated builds by linking Docker Hub to a GitHub or Bitbucket repository.
32. What is Docker Hub’s rate limit policy, and how does it affect users?
    • Explain the limitations on image pulls for free accounts and the options available to mitigate these limits.
33. How do you tag and push images to Docker Hub?
    • Walk through the steps to tag an image (docker tag) and push it to Docker Hub (docker push).
34. What is the purpose of Docker Hub organizations and teams?
    • Explain how organizations and teams in Docker Hub allow for better collaboration and control over repositories.
35. How do you search for and pull images from Docker Hub?
    • Describe how to use docker search and docker pull to find and download images from Docker Hub.
36. How does Docker Hub integrate with other CI/CD tools?
    • Discuss integrations with Jenkins, CircleCI, and other CI/CD pipelines for automated image building and deployment.
37. What are Docker Hub Webhooks, and how are they used?
    • Explain how webhooks can trigger events, such as builds or deployments, when new images are pushed to Docker Hub.

These questions cover the core concepts and advanced topics related to Docker, Docker Compose, Swarm, Docker Trusted Registry (DTR), and Docker Hub. By being familiar with these questions, you can demonstrate a deep understanding of Docker containerization and its ecosystem.