Top 20 Common Docker Interview Questions and How to Answer Them

Docker is a software platform that enables us to package and deploy applications consistently. Docker containers are self-contained packages containing all the elements necessary to run an application, such as code, runtime, system tools, libraries, and settings. This approach empowers us to develop and run applications in diverse environments, irrespective of the underlying operating system or infrastructure.

We use Docker as a widely adopted tool for developing and deploying containerized applications. It's utilized by businesses ranging from small startups to large enterprises. Docker aids us in enhancing our development procedures, enhancing application portability, and cutting down costs.

Following are some important Docker Interview Questions:

1. Explain the difference between Docker Vs VM (Virtual Machine)

1. What is Docker?

   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 to have a product-ready application

2. What is the advantage of Docker over hypervisors?

   Docker is lightweight and more efficient in terms of resource uses because it uses the host underlying kernel rather than creating its own hypervisor.

3. What is Docker's image?

   A Docker image is a lightweight, executable package that includes everything needed to run an application: code, runtime, system tools, system libraries, etc.

   Docker images are built using a Dockerfile, which is a text file that contains a set of instructions for building the image. Docker images can be stored in a Docker registry, which is a central repository where users can share and download images.

   Docker images are portable and can be used to run applications on any platform that has Docker installed. This makes Docker images an ideal way to package and deploy applications, as they can be quickly and easily moved between environments without the need to modify the application code.

4. What is a Docker container?

   • Docker containers include the application and all of its dependencies, but share the kernel with other containers, running as isolated processes in user space on the host operating system. Docker containers are not tied to any specific infrastructure: they run on any computer, on any infrastructure, and in any cloud.

   • Docker containers can be created from Docker images, which are pre-built images that contain everything needed to run an application we can use Docker images that are present on the Dockerhub.

• Docker containers can be started, stopped, and paused as needed.

1. What is a Docker hub?

   Docker Hub is like a virtual storage space in the cloud. It helps you connect to your code, create and test custom images, save your creations, and easily send them to your servers via Docker cloud. It acts as a hub where you can find, share, and manage container images. Plus, it simplifies teamwork, allowing users and teams to collaborate smoothly and automates the workflow during the development process.

2. What is Docker Swarm?

   • Docker Swarm is a native clustering and orchestration tool for Docker containers.

   • It allows you to create and manage a cluster of Docker nodes, turning them into a single virtual system. This cluster can then be used to deploy, scale, and manage containerized applications seamlessly.

   • Docker Swarm simplifies the process of deploying and managing containers across multiple hosts, providing high availability and fault tolerance for your applications. It is designed to be easy to use, making it an excellent choice for orchestrating containerized applications in a production environment.

1. What is Dockerfile used for?

   A Dockerfile is a text document that contains a set of instructions for building a Docker image. Each instruction in a Dockerfile tells Docker what to do, such as what software to install, what files to copy, and what environment variables to set.

   Here is an example of a simple Dockerfile that builds a Docker image for a Python web application, all the steps are immediate layers:

   FROM python:3.8

   WORKDIR /app

   COPY requirements.txt .

   RUN pip install -r requirements.txt

   COPY . .

   CMD ["python", "app.py"]

2. Can we use JSON instead of YAML for our Docker-Compose file?

   YES, you can very comfortably use JSON instead of the default YAML for your Docker compose file. In order to use JSON file with composing, you need to specify the filename to use as the following:

docker-compose -f docker-compose.json up

1. How to create a Docker container?

   We can create a Docker container out of any specific Docker image of your choice and the same can be achieved using the command given below:

   docker run -d <image name>

   The command above will create the container. In order to check whether the Docker container is created and whether it is running or not, you could make use of the following command. This command will list out all the Docker containers along with their statuses on the host that the Docker container runs.

docker ps -a

1. How to stop and restart the Docker container?

   The following command can be used to stop a certain Docker container with the container id as

docker stop CONTAINER_ID

The following command can be used to restart a certain Docker container with the container id as

docker restart CONTAINER_ID

1. How far do Docker containers scale?

   Some of the biggest names on the internet, such as Google and Twitter, use Docker for their web services. Docker allows these services to handle an enormous number of containers, ranging from hundreds of thousands to millions, all running simultaneously. The only limit is the capacity of the operating system and memory on the hosting servers. These containers work together to host various applications, making them a powerful and scalable solution for web deployments.

2. What platforms does Docker run on?

   Docker is currently available and can run on the following Cloud environment setups given below:

   • Amazon EC2

   • Google Compute Engine

   • Microsoft Azure

   • Rackspace

1. How many containers can run per host?

   • The number of containers that Docker can host depends on the specific environment. It's all about what the environment can handle in terms of application size and available resources like CPU and memory.

   • The containers are clever because they create new CPUs as needed, which helps in efficient resource utilization.

   • These containers are incredibly lightweight and only stick around for as long as the task they're performing, making them efficient.

1. What are the differences between the ‘docker run’ and the ‘docker create’?

   The main difference between docker run and docker create is that docker run creates a container and starts it immediately, while docker create only creates a container without starting it.

   • docker run is the most commonly used command for creating and running containers. It takes an image name or ID as its first argument, followed by any other options and commands that you want to run inside the container. For example, to run a web server container, you would use the following command:

     docker run -p 80:80 nginx

1. What are the various states that a Docker container can be in at any given point in time?

   There are four states that a Docker container can be in, at any given point in time. Those states are as given as follows:

   • Running
   • Paused
   • Restarting
   • Exited

2. Can you remove a paused container from Docker?

   No, it is not possible to remove a container from Docker that is just paused. It is a must that a container should be in the stopped state before it can be removed from the Docker container.

3. What is the best way of removing containers - ‘docker rm -f’ or ‘docker stop’ then followed by a ‘docker rm’?

   The best and the preferred way of removing containers from Docker is to use the ‘docker stop’, as it will allow sending a SIG_HUP signal to its recipients giving them the time that is required to perform all the finalization and cleanup tasks. Once this activity is completed, we can then comfortably remove the container using the ‘docker rm’ command from Docker and thereby update the docker registry as well.

4. Difference between Docker Image and container?

   A Docker image is a lightweight, standalone, executable package that contains everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and configuration files. It's a snapshot of a filesystem that also includes metadata about the image and its dependencies. Think of it as a blueprint or a template for creating a container. Images are static and unchangeable. They are used to create containers and can be stored and shared through Docker registries.

   On the other hand, a Docker container is a runnable instance of a Docker image. It's a lightweight and portable executable package that includes the application and everything it needs to run, including the operating system. Containers are created from images and are the real runtime entities. They encapsulate an application and its dependencies and isolate it from the underlying system. Containers are dynamic and can be started, stopped, moved, and deleted. They provide consistency across different environments, ensuring that an application runs the same way on a developer's laptop as it does in production.

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