Docker is one of the most popular containerization tools used by software developers and system administrators worldwide. It is a platform that automates the deployment, scaling, and management of applications. As a result, many tech companies are looking for candidates with a solid understanding of Docker. Here is a list of 50 Docker interview questions with answers to help you prepare for your upcoming interview.
1. What is Docker?
Docker is an open-source platform that automates the deployment, scaling, and management of applications. It uses containerization technology to bundle an application and its dependencies into a single unit.
2. What is a Docker Container?
A Docker container is a lightweight, standalone, and executable software package that includes everything needed to run an application: the code, runtime, system tools, system libraries, and settings.
3. What is the difference between a Docker image and a Docker container?
A Docker image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software. A Docker container is a runtime instance of a Docker image.
4. How is Docker different from Virtual Machines?
Docker containers share the host system’s OS kernel, while Virtual Machines (VMs) have their own Guest OS. As a result, containers start faster and require less memory than VMs.
5. What is Docker Hub?
Docker Hub is a cloud-based repository where Docker users and partners create, test, store, and distribute Docker container images.
6. What is a Dockerfile?
A Dockerfile is a text file that Docker reads to automatically build an image. It contains instructions and commands a user could call on the command line to assemble an image.
7. What is Docker Compose?
Docker Compose is a tool for defining and managing multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services.
8. How do you create a Docker container?
You can create a Docker container using the docker run command followed by the name of the image.
9. What is the purpose of the EXPOSE command in a Dockerfile?
The EXPOSE command informs Docker that the container listens on the specified network ports at runtime.
10. What is Docker Swarm?
Docker Swarm is a clustering and orchestration tool for Docker. It allows you to manage multiple containers deployed across multiple host machines.
11. What is the difference between Docker Swarm and Kubernetes?
Both Docker Swarm and Kubernetes are container orchestration tools, but they differ in complexity and functionality. Kubernetes offers more features but is more complex, while Docker Swarm is simpler and more tightly integrated with Docker.
12. How do you remove a Docker image?
To remove a Docker image, you can use the docker rmi command followed by the image ID or name.
13. What is a Docker volume?
Docker volumes are the preferred way to persist data generated by and used by Docker containers. Volumes are completely managed by Docker.
14. What are Docker namespaces?
Docker uses namespaces to provide the isolated workspace called the container. When you run a container, Docker creates a set of namespaces for that container.
15. How can you copy files from Docker host to Docker container?
You can copy files from the Docker host to a Docker container using the docker cp command.
16. What is Docker Daemon?
The Docker daemon is a service that runs on your host operating system. It listens for Docker API requests and manages Docker objects such as images, containers, networks, and volumes.
17. What is Docker networking?
Docker networking is a way for Docker containers to communicate with each other and with other systems.
18. How do you tag a Docker image?
To tag a Docker image, you use the docker tag command. This command creates a tag that refers to the image ID.
19. What is Docker Engine?
Docker Engine is the underlying client-server technology that builds and runs containers using Docker’s components and services.
20. How can you run multiple commands in a Docker container?
You can run multiple commands in a Docker container using a single RUN statement and chaining commands using the && operator.
Questions 21-50 will focus on more advanced topics, requiring deeper understanding and practical knowledge of Docker.
21. What is the difference between CMD and ENTRYPOINT in a Dockerfile?
CMD is used to provide defaults for an executing container, while ENTRYPOINT allows you to configure a container that will run as an executable.
22. How can you link containers in Docker?
You can link containers in Docker using the --link flag in the docker run command.
23. How does Docker handle logging?
Docker has a logging driver which can direct container output to various places, such as a log file or stdout.
24. How can you see the IP address of a running Docker container?
You can see the IP address of a running Docker container using the docker inspect command.
25. What is the difference between docker save and docker export?
docker save is used to persist a Docker image, while docker export is used to persist a Docker container.
26. What is Docker’s layered architecture?
Docker uses a layered architecture for images. Each instruction in the Dockerfile creates a new layer in the image.
27. How can you extend a Docker image?
You can extend a Docker image by creating a new Dockerfile, using the FROM command followed by the name of the image you want to extend, and then adding your own instructions.
28. What is Docker overlay network?
Docker overlay network is a network that spans across multiple Docker hosts and enables swarm services to communicate with each other.
29. How do you handle sensitive data in Docker?
Sensitive data in Docker can be handled using Docker secrets. Secrets are encrypted during transit and at rest in a Docker swarm.
30. How can you list all Docker containers?
You can list all Docker containers using the docker ps -a command.
31. How can you start and stop a Docker container?
You can start a Docker container using the docker start command and stop it using the docker stop command.
32. How can you handle persistent storage in Docker?
Persistent storage in Docker can be handled using Docker volumes, which survive container restarts and can be shared among multiple containers.
33. How can you manage Docker remotely?
You can manage Docker remotely by exposing the Docker API over a network and connecting to it using the Docker command-line interface or REST API.
34. What is a Docker service?
In Docker Swarm, a service is the definition of the tasks to execute on the manager or worker nodes. It is the central structure of the swarm system and the primary root of user interaction with the swarm.
35. How do you run a Docker container in detached mode?
To run a Docker container in detached mode, you use the -d option in the docker run command.
36. What are the Dockerfile best practices?
Dockerfile best practices include minimizing the number of layers, using multi-stage builds, not installing unnecessary packages, and using .dockerignore files.
37. How can you monitor Docker in real-time?
You can monitor Docker in real-time using tools like docker stats for container performance and Docker’s native logging driver.
38. How can you scale Docker containers?
You can scale Docker containers using Docker Compose with the docker-compose scale command, or using Docker Swarm with the docker service scale command.
39. What is a Docker node?
In Docker Swarm, a node is an instance of the Docker engine participating in the swarm. Nodes can be either manager nodes or worker nodes.
40. What is Docker trusted registry?
Docker Trusted Registry (DTR) is Docker’s commercial-grade, on-premises container image storage solution. It is installed behind your firewall, in your data centers, or on virtual private cloud networks.
41. How can you deploy a Docker app to production?
You can deploy a Docker app to production by creating a Dockerfile, building an image from that Dockerfile, pushing the image to a registry, and then pulling and running the image on your production servers.
42. How do you clean up unused Docker images, containers, and volumes?
You can clean up unused Docker images, containers, and volumes using the docker system prune command.
43. How can you debug a Docker container?
You can debug a Docker container using the docker logs command, or by using docker exec -it <container-id> /bin/bash to start a new shell in the container.
44. How can you limit memory usage in a Docker container?
You can limit memory usage in a Docker container using the -m or --memory flag in the docker run command.
45. How can you use Docker in a Continuous Integration/Continuous Deployment (CI/CD) process?
In a CI/CD process, Docker can be used to build, test, and deploy applications in an isolated and consistent environment. Docker images can be built in a CI process and then pushed to a registry, from which they can be pulled and deployed in a CD process.
46. How can you specify a Docker network when running a container?
You can specify a Docker network when running a container using the --network flag in the docker run command.
47. How can you update a running Docker service?
You can update a running Docker service using the docker service update command.
48. How can you manage data in Docker?
You can manage data in Docker using Docker volumes, bind mounts, or tmpfs volumes. These allow you to persist and share data among containers.
49. How do you inject environment variables in Docker?
You can inject environment variables in Docker using the -e flag in the docker run command.
50. What is a Docker stack?
A Docker stack is a group of interrelated services that share dependencies, and can be orchestrated and scaled together in Docker Swarm mode.
Preparing for a Docker interview can be challenging, but with these questions and answers, you should be well on your way to acing your interview. Remember, it’s not just about knowing the answers to these questions, but also understanding the underlying concepts and being able to apply them in real-world situations. Good luck!