35 Docker Interview Questions and Answers (2025)

For many years, Docker has been the most wanted platform. It is one of the preferred tools for handling applications. It handles most aspects, from application building to scaling, and offers a safe, secure environment for individual and organizational needs. With huge potential, high preferences, and rising demands, 24,000+ active Docker jobs exist.

Are you eyeing such roles due to promising career opportunities? If yes, or if you are considering it as an option, you are at the right place. Whatever your current expertise level with Docker is, this interview guide will help you familiarise yourself with the concepts or brush up on them. Scroll to learn from the Docker interview questions and answers and get interview-ready.

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Docker Overview

Docker is an open-source platform for building, testing, deploying, updating, managing, and scaling applications. It streamlines operations by offering software packed into standardised and executable units called containers. These units comprise essential components required to run the software. Examples of components include runtime, libraries, system tools, and code.

Basic Docker Interview Questions

The fundamental level interview questions on Docker are as follows:

1. What is Docker, and why is it used?

Docker is an open-source platform for application development, from building to scaling. It is preferred for its ability to accelerate the process of building, sharing, and running applications without requiring environment configuration or management.

Docker integrates with existing software development tools, optimizing the workflow. The platform is also compatible with multiple cloud platforms.

2. What is a Docker container?

A Docker container is a standard software packaging unit that contains code and other dependencies such as system libraries, runtime, system tools, and settings. These speed up the application's running across various environments. In simple terms, Docker can be considered an executable, standalone, and lightweight package.

3. How do you create a Docker container?

A Docker container can be created using the ‘docker container create’ command in the system command. This command establishes the new container from the specified image without starting it immediately. The newly created container is generated in a ‘created’ state, writable, and open to running specific commands.

4. How does Docker differ from a virtual machine?

Docker is a software platform for creating and running Docker containers, while a Virtual Machine refers to the physical machine that runs an operating system.

Docker boots in a few seconds, uses an execution engine, requires less memory, and involves a complex usage method. VM takes a few minutes to boot, uses a hypervisor, is less memory efficient, and is easy to use.

5. What is a Docker image?

Docker images are read-only templates in the form of a blueprint or snapshot. They contain guidelines or instructions for creating a container. These images are standalone and executable files that comprise the dependencies, libraries, and files essential for the container.

Docker images are shareable and portable and can be stored in registries to track projects, user group access, complex software architectures, and others.

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6. How do you push and pull Docker images?

Docker images are stored in Docker Hub. To obtain the image or pull it, the required command is

docker pull rocker/verse

To push the image to the newly created repository, the appropriate command is

docker push yourhubusername/verse_gapminder

7. What is a Dockerfile?

A Dockerfile is a text document comprising all the commands required for image assembly. Users call these commands. Examples of Dockerfile commands include ‘ADD’, ‘ARG’, ‘COPY’, ‘LABEL’, and many more.

The commands aren’t case-sensitive, but being in uppercase helps easy identification of the arguments used in the command. Dockerfiles are used to run the instructions by Docker, and they must begin with a ‘FROM’ instruction.

8. What is a Docker registry?

The Docker registry is the centralized storage, management, and distribution system for container images. It comprises Docker repositories that contain different versions of the images. Images from the registry can be accessed through push and pull commands.

9. What is Docker Hub?

Docker Hub is a hosted registry solution available directly from Docker Inc. It provides public and private repositories and solutions like automated build and integrated source control.

The public repository is open for access, and the image names are as follows: organization/user name. Private repositories are private to the repository creator or organization member. The Docker Hub images are verified and secured for safe usage.

10. What is Docker Compose?

Docker Compose is a tool for defining and running multi-container applications. It allows projects to run with multiple containers from a single source: a YAML configuration file.

Docker Compose allows you to manage various services, data volumes, and networks easily. It is also compatible with working in all environments, such as production, staging, development, testing, and CI workflows.

Intermediate Docker Interview Questions

The Docker interview questions for the intermediate level are as follows:

11. How do you create a Docker network, and why would you need it?

A Docker network can be created through the following command:

docker network create [OPTIONS] NETWORK

The Docker network connects different services running in separate containers. Its functionality is essential, especially in scenarios requiring service communication while being decoupled. The preference can further be owed to features like flexible network models, automatic service discovery, security, isolation, and dynamic scaling.

12. What is the difference between Docker run and Docker start?

Docker’s run command creates and starts a new container from a specific image. Running here is done for an already started container that requires the command to execute the main process.

The start command works to start a stopped container, which requires beginning to execute the main process. The container may have stopped for various reasons, such as using the stop command, memory overconsumption, or the operating system.

13. What are Docker volumes?

Docker volumes are the container storage and management components of Docker. It can be easily created through the command ‘docker volume create’ or via Docker itself during service or container creation.

Docker volumes are preferred because they are easy to manage through Docker CLI commands or Docker API, secure sharing across multiple containers, and operability on both Linux and Windows containers.

14. How do you update an existing Docker image?

Updating the existing Docker image is possible through the following steps:

• Check the current version by using the command ‘docker images’
• Pull the latest Docker image by downloading the new version of the image
• Use the ‘latest’  tag if unsure about the host system default
• You can also update the image to a specific version using the command ‘docker pull [image]:[version-number]’
• Stop and remove the container based on the old image. It requires finding the outdated container with the command ‘docker ps’
• Stop the container using the command ‘docker stop [container-id-or-name] ’ and remove it with the command ‘docker rm [container-id-or-name]’
• Create a new container using the run command, which is ‘docker run [options] [image] [commands]’

Note: You can check whether a container will run the new Docker image by listing the containers using the above commands.

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

Multi-stage Docker optimizes the Dockerfiles and images without compromising the ease of readability and maintenance. It converts the image-building process into multiple stages, which allows for the inclusion of the necessary dependencies per the final application's functionality. It offers memory and time efficiency.

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16. How can you improve the performance of Docker containers?

Improving the performance of Docker containers is possible in the following ways:

• Modify the frequency and size of Docker images when running a Docker container
• Run containers as non-root users whenever possible to reduce attacks
• Make use of multi-stage build, which is to be done by using two or more ‘FROM’ statements
• Set resource containers to limit container resources by using the ‘-- cups’ and ‘-- memory’ flags
• Preload the image cache and use lightweight base images
• Use monitoring tools to optimize container health

17. How do you share data between containers?

Follow the mentioned steps to share data between containers:

• Create an independent volume named Datavolume1 using the command ‘docker volume create --name DataVolume1’
• Create a new container and attach Datavolume1 using the command ‘docker run --rm -ti -v DataVolume1:/datavolume1 ubuntu’
• Verify its persistence using the command ‘cat /datavolume1/Example1.txt’
• Use the command to create a new container and new volume ‘docker run -ti --name=Container2 -v DataVolume2:/datavolume2 ubuntu’
• Add data to the volume with the command ‘echo ‘Example2" > /datavolume2/Example2.txt

cat /datavolume2/Example2.txt’

• Exit the container and restart. Then verify the volume has mounted and data’s persistence using the command ‘cat /datavolume2/Example2.txt’
• Remove the container using ‘docker rm d0d2233b668eddad4986313c7a4a1bc0d2edaf0c7e1c02a6a6256de27db17a63’
• Check the persistence by listing the volumes with the command ‘docker volume ls’
• Use ‘docker volume rm DataVolume2’ to remove the second volume
• Add the data image with the command ‘docker run -ti --rm -v DataVolume3:/var ubuntu’ and then exit and check the volume contents
• Create container4 and datavolume4 using the previous command
• Now create a file with the text ‘echo "This file is shared between containers" > /datavolume4/Example4.txt’ followed by exiting the container 
• Again, create container5 to mount the volume from container4 and check the data persistence
• Append the text from container5 and exit the container
• Restart container4 with the command ‘docker start -ai Container4’ and check the changes using ‘lcat /datavolume4/Example4.txt’ and exit
• Make the volume read-only using ‘docker run -ti --name=Container6 --volumes-from Container4:ro ubuntu’
• Check the status by removing the example file using the command ‘rm /datavolume4/Example4.txt’
• Exit and clean the containers and volume 

18. How does Docker handle container orchestration?

In addition to other container orchestration tools, Docker can perform the function with its built-in tool, Docker Swarm. It is a user-friendly and simple way to orchestrate, requiring interactions similar to regular Docker CLI commands.

Swarm allows service discovery, replica count, declarative scaling, and rolling updates. Docker Compose is another widely used method of handling container orchestration.

19. What is the use of docker-compose.yml?

Docker-compose.yml is used for container orchestration. It defines the required images, exposed ports,  access to host filesystems, and other configurations to run multi-counter applications efficiently.

20. How would you manage multiple containers using Docker Compose?

Docker Compose eases the management of multiple containers by defining the entire multi-container application in a single YAML file named compose.yml. It allows for the running of containers in a specific order and the management of network connections easily. It also enables scaling individual services up or down based on demand.

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Advanced Docker Interview Questions

The Docker interview questions for experienced professionals are as follows:

21. How does Docker handle orchestration, and what tools are available for it?

Docker can handle orchestration through both built-in tools and external tools. The built–in tools include Docker Swarm and Docker Compose. The commonly used external tool for orchestration is Kubernetes.

22. What is Docker content trust, and how is it used?

Docker content trust allows verification of the pulled or deployed images from the registry server. It verifies the publisher and integrity of the received data and offers the ability to use digital signatures for sent and received data. The digital signatures are added by the publishers, like organisations or individuals, and signed and stored on the Docker Notary server of choice.

23. Explain the concept of namespaces in Docker.

Namespaces are an important concept in Docker that provides an isolated workspace for the container. Dockers creates a set of namespaces for the container being run. They are features of the Docker Linux kernel that isolate kernel resources, allowing each set of processes to see only their environment.

There are various types of namespaces, including user namespace, network namespace, process ID (PID) namespace, interprocess communication (IPC) namespace, and others.

24. How does Docker achieve container isolation?

Container isolation in Docker is possible through Linux features such as control groups (cgroups), kernel namespaces, and secure computing mode (seccomp). For instance, Docker configures cgroups by creating a new cgroup hierarchy and assigns them the container’s processes, Kernel namespaces allow a set of processes an isolated view of resources, and seccomp works by allowing filtering the system calls made by a process.

25. What is the role of cgroups in Docker?

Control groups, or cgroups, are a feature of the Linux kernel used to limit resource availability and allocate system resources such as memory and CPU for Docker containers. They help avoid ‘noisy neighbour’ issues and are used by other containerization tools. Docker uses cgroups for container isolation, and each container is associated with its own cgroups.

26. How do you configure persistent storage in Docker?

Docker ensures data resilience through container volumes. It involves creating a volume and mounting it into the container with a command. The volume remains accessible even after deleting the container. The volumes for persistent storage comprise their lifecycle and can expand depending on the type of application and data being used.

27. What is a Docker registry mirror, and why would you use it?

Registry mirror refers to the local duplicate of the public registry, which mirrors or replicates its file structure.  It pulls the image from the public registry and stores it before providing it to the user. Then the next request involves producing the image from the legal registry mirror.

The Docker registry mirror protects against public registry outages, performs image vulnerability scanning, accelerates pod creation, and avoids limits on accessing the public registry.

28. What is the Docker security scan, and how does it work?

Docker security scans assess vulnerabilities in container images. Depending on the scanning tool, they can detect a variety of risks. The scanning evaluates the packages and other dependencies defined in the container image file to identify vulnerabilities.

Docker security scans alert the user if a vulnerability is found, which can be further handled by updating the Docker image to the latest version. Docker generally has built-in scanning tools inside the registry, while image scanners like Clair and Anchore can also be used for scanning.

29. How can Docker improve Continuous Delivery (CD)?

Docker contributes to better Continuous Delivery (CD) by streamlining and accelerating the deployment process. It integrates well with source control management and the integration tool. The feature also automatically creates an image when the user submits the code.

Docker also allows developers to build and test applications in parallel, thus reducing deployment time while minimizing errors. 

30. What are Docker Plugins, and how are they used?

Docker Plugins are extensions that enhance the capabilities of the Docker engine. There are specific types of Docker Plugins, such as volume, authorization, and network plugins. The volume allows volume persistence across various Docker hosts, while the network plugin allows network plumbing. The authorization plugin handles Docker daemon requests.

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Scenario-based Docker Interview Questions

Familiarize yourself with some real-life interview questions on Docker, which are listed as follows: 

Scenario 1: You need to deploy a microservices application using Docker. How would you structure the Dockerfiles and Docker Compose to manage multiple containers?

Firstly, set up the project that will comprise Angular as the front-end client, Node.js API to handle business backend operations, MongoDB to store its data, Java API for managing book-related business logic and processes, MySQL as a relational database, and Nginx as a reverse proxy to route client requests.

Now, create Dockerfiles for each service. It will be as follows:

• Angular and Nginx Dockerfile created with a multi-stage build

Build the Angular app
FROM node:14 AS web-build
WORKDIR /usr/src/app
COPY ./ ./client
RUN cd client && npm install && npm run build — prod
Serve the app with Nginx
FROM nginx:latest
COPY - from=web-build /usr/src/app/client/dist/client/ /usr/share/nginx/html
COPY nginx.conf /etc/nginx/conf.d/default.conf
EXPOSE 4200
Node.js API

• Dockerfile to build and run the Node.js API

FROM node:14 AS nodeapi-build
WORKDIR /usr/src/app
COPY ./ ./nodeapi/
RUN cd nodeapi && npm install
FROM node:14
WORKDIR /usr/src/app/
COPY - from=nodeapi-build /usr/src/app/nodeapi/ ./
EXPOSE 5000
CMD ["/bin/sh", "-c", "cd /usr/src/app/ && npm start"]

• Dockerfile to compile and run the Java API

FROM openjdk:8 AS BUILD_IMAGE
WORKDIR /usr/src/app/
RUN apt update && apt install maven -y
COPY ./ /usr/src/app/
RUN mvn install -DskipTests
FROM openjdk:8
WORKDIR /usr/src/app/
COPY - from=BUILD_IMAGE /usr/src/app/target/book-work-0.0.1-SNAPSHOT.jar ./book-work-0.0.1.jar
EXPOSE 9000
ENTRYPOINT ["java","-jar","book-work-0.0.1.jar"]

Now, using Docker Compose to define and run the services. The file will comprise the services along with relevant dependencies and their interaction.

version: “3.8”

services:

client:

build:

context: ./client

ports:

- "4200:4200"

container_name: client

depends_on:

- api

- webapi

api:

build:

context: ./nodeapi

ports:

- "5000:5000"

restart: always

container_name: api

depends_on:

- nginx

- emongo

webapi:

build:

context: ./javaapi

ports:

- "9000:9000"

restart: always

container_name: webapi

depends_on:

- emartdb

nginx:

restart: always

image: nginx:latest

container_name: nginx

volumes:

- "./nginx/default.conf:/etc/nginx/conf.d/default.conf"

ports:

- "80:80"

emongo:

image: mongo:4

container_name: emongo

environment:

- MONGO_INITDB_DATABASE=epoc

ports:

- "27017:27017"

emartdb:

image: mysql:8.0.33

container_name: emartdb

ports:

- "3306:3306"

environment:

- MYSQL_ROOT_PASSWORD=emartdbpass

- MYSQL_DATABASE=books

It will be followed by building the services, starting the application, and pushing the images to Docker Hub.

Scenario 2: You have a Docker container consuming too much memory. How would you diagnose the issue and solve it without restarting the container?

The terminal's ‘docker stats’ command offers insight into CPU, memory, network, and disk usage. For deeper insights, using the Resource Usage extension is a better option. It helps identify the projects that use the most resource-intensive containers or Docker Compose. The extension further assists in monitoring the resource usage change over time and provides basic command-based information.

Update the memory limit and enable kernel memory accounting to solve the issue of running containers. Possible solutions for containers that are not running include memory reservation, setting memory limits with the—memory Flag, and using the—memory-swap flag.

Scenario 3: You want to scale a web application running in Docker. Describe how you would scale it horizontally using Docker Swarm or Kubernetes.

To scale the single service in a web application running in Docker horizontally, use the command ‘docker service scale <servicename>=<replicacount>’. It can be checked by running the ‘docker service ls’ command. Scaling operations for single and multiple services can be achieved through the ‘docker service update’ command.

Scenario 4: Your development team is working with multiple environments (development, staging, production) using Docker. How would you handle environment-specific configurations in Docker?

Setting environment-specific configurations in Docker can be done using the environment attribute in compose.yaml and using .env files along with the env_file attribute. Further, temporary variables can also be set with the command ‘docker compose run --env’.

Scenario 5: You need to secure sensitive data inside Docker containers. What strategies would you use to ensure security, especially when running containers in a production environment?

Top strategies for securing running containers in a production environment include limiting container privileges, using kernel security features, setting a non-root user in Dockerfiles, disabling SSH access, using TLS encryption, scanning images for vulnerabilities, limiting resource usage, using namespace remapping, and other such methods.

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Tips to Prepare for a Docker Interview

Preparing for an interview requires focus on the following aspects:

• Gain insights into fundamental concepts of Docker, such as Docker images, volumes, and Dockerfiles
• Master core components such as Docker Hub, Docker Compose, and others
• Aim for hands-on experience with practical projects
• Be well-versed in syntax and commands used in Docker
• Familiarize yourself with Docker networking
• Work on existing challenges in working with Docker by hunting for solutions or the best-fit approach

Conclusion

Docker's ease of use and convenience have contributed to its widespread popularity. Job profiles like Docker Developer, DevOps Engineer, Cloud Engineer, and others are expanding. Grabbing the opportunity early is possible with in-depth knowledge and the ability to exhibit it to the interviewer. This capability comes with practice, and to assist you with the preparation, we have created this interview guide to familiarize you with essential questions and answers across different levels.

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