Top Answers to Docker Interview Questions


With years passing by, Docker has been showing exponential growth in terms of the rise in job openings. Professionals who have mastered the skills in Docker are in high demand today. The Docker Interview Questions and Answers blog is mainly created to give you an idea of the type of questions that are asked during Docker job interviews. So, take advantage of the latest and high-paying Docker job opportunities by preparing for your interview with these interview questions:

Q1. What is Docker?
Q2. What is the benefit of using a Docker over a Hypervisor?
Q3. What are the unique features of Docker over other containerization technologies?
Q4. What is a Docker image?
Q5. What is a Docker container?
Q6. What is a Docker Hub?
Q7. What is a Docker Swarm?
Q8. What is the use of a Dockerfile?
Q9. Is it possible to use JSON instead of YAML for Docker Compose?
Q10. Tell us how you have used Docker in your past position.

These Docker interview questions are broadly divided into major categories such as:
1. Basic

2. Intermediate

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Top Docker Interview Questions and Answers

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Basic Interview Questions

1. What is Docker?

Criteria Docker Virtual Machines
Use of OS All containers share the host OS Each VM runs on its own OS
Startup time Very fast Slow
Isolation Process-level isolation Full isolation
Security Low High

We can define Docker as a containerization platform that combines all our applications in a package so that we have all the dependencies to run our applications in any environment. This means, our application will run seamlessly on any environment, and this makes it easy for having a product-ready application. What Docker does is wrap the software needed in a file system that has everything for running the code, providing the runtime and all the necessary libraries and system tools. Containerization technology like Docker will share the same operating system kernel with the machine, and due to this it is extremely fast. This means that we have to run Docker only at the beginning and after that, since our OS is already running, we will have a smooth and seamless process.

2. What is the benefit of using a Docker over a Hypervisor?

Though Docker and Hypervisor might do the same job overall, there are many differences between them in terms of how they work. Docker can be thought of as lightweight since it uses very fewer resources and also the host kernel rather than creating it like a Hypervisor.

3. What are the unique features of Docker over other containerization technologies?

Some of the most important and unique features of Docker are as follows:

  • We can run our Docker container either on our PC or on our enterprise IT system.
  • Along with the Docker Hub, which is a repository of all containers, we can deploy and download all our applications from a central location.
  • We can even share our applications with the containers that we create.

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4. What is a Docker image?

A Docker image helps in creating Docker containers. We can create the Docker image with the build command; due to this, it creates a container that starts when it begins to run. All the Docker images are stored in the Docker registry such as the public Docker registry. These have minimal amounts of layers within the image so that there is a minimum amount of data on the network.

5. What is a Docker container?

It is a comprehensive set of applications including all its dependencies which share the same OS kernel, along with the other containers running in separate processes within the operating system in a user space. Docker is not tied to any IT infrastructure, and thus it can run on any computer system or on the cloud. We can create a Docker container using Docker images and then run it, or we can use the images that are already created in the Docker Hub. To simplify things, let us say that the Docker containers are just runtime instances of the Docker image.


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Intermediate Interview Questions

6. What is a Docker Hub?

We can think of Docker Hub as a cloud registry that lets us link the code repositories, create the images, and test them. We can also store our pushed images, or we can link to the Docker Cloud, so that the images can be deployed to the host. We have a centralized container image discovery resource that can be used for the collaboration of our teams, automating the workflow and distribution, and changing management by creating the development pipeline.

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7. What is a Docker Swarm?

We can think of a Docker Swarm as the way of orchestrating the Docker containers. We will be able to implement Dockers in a cluster. We can convert our Docker pools into a single Docker Swarm for easy management and monitoring.

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8. What is the use of a Dockerfile?

A Dockerfile is a set of specific instructions that we need to pass on to Docker so that the images can be built. We can think of the Dockerfile as a text document which has all the commands that are needed for creating a Docker image. We can create an automated build that lets us execute multiple command lines one after the other.

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9. Is it possible to use JSON instead of YAML for Docker Compose?

We can use JSON instead of YAML for a Docker Compose file. When we are using the JSON file for composing, we have to specify the filename with the following command:

docker-compose -f docker-compose.json up

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10. Tell us how you have used Docker in your past position.

This is a question wherein we could bring upon our whole experience with Docker and any other Container technologies we have used prior to Docker. We could also explain the ease that this technology has brought in the automation of the development-to-production life cycle management. We can also discuss about any other integrations that we might have worked, along with Docker, such as Puppet, Chef, or even the most popular of all technologies—Jenkins. If we do not have any experience with Docker but we have it with similar tools from this space, we could convey the same and also show our interest in learning this leading containerization technology.

11. What is the process for creating a Docker container?

We can use any specific Docker image for creating a Docker container using the below command:

docker run -t -i command name

This command not only creates the container but also starts it for us. If we want to check whether the Docker container has been created or not, then we need to have the following command that will list all the Docker containers, along with the host on which the Docker containers run:

docker ps -a

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12. What is the process for stopping and restarting a Docker container?

To stop a Docker container, we need to use the following command:

docker stop CONTAINER_ID

To restart a Docker container, we need to use the following command:

docker restart CONTAINER_ID

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13. How do you scale your Docker containers?

Docker containers can be scaled to any level, starting from a few hundreds to even thousands or millions of containers. The only condition is that the containers need the memory and the OS all the time, and there should not be a constraint on these when the Docker is getting scaled.

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