📍Introduction

As Docker continues to grow in popularity, the demand for skilled Docker developers has increased. In this blog, we will explore some common docker interview questions that might encounter when interviewing for a Docker-related role. These questions will cover a range of topics, including the basic of Docker, Dockerfile, and Docker Compose. If you looking to brush up on your skills, this blog will help you prepare for your next Docker interview.

🔹What is the difference between an Image, Container and Engine?

1. Image: An image is a read-only template that contains the instructions for creating a Docker container. An image is essentially a snapshot of a Docker container at a specific point in time and it includes all the necessary files, dependencies, and configurations needed to run an application. Docker images can be stored and distributed in a Docker registry, such as Docker Hub.

2. Container: A container is a running instance of an image. When you start a container, Docker creates a writable layer on top of the read-only image, allowing you to modify the container's state and data without affecting the underlying image. Containers are lightweight, portable, and isolated from each other, making it possible to run multiple applications on the same host without conflicts.

3. Engine: The Docker engine is the core component of the Docker platform. It's responsible for managing the containers and images, scheduling container workloads, and communicating with the host operating system to run containers. The Docker engine consists of a daemon process that runs in the background, a REST API that allows you to interact with the daemon, and a command-line interface (CLI) that provides a user-friendly way to manage Docker resources.

🔹What is the difference between the Docker Command COPY vs ADD?

COPY is a simpler instruction that only supports copying local files and directories into the Docker image, it does not support extracting the tar file and copying files from remote URLs. ADD on other hand, supports both copying local files and directories as well as remote URLs. ADD command automatically extract the tar file.

🔹What is the Difference between the Docker command CMD vs RUN?

1. RUN: The RUN instruction is used to execute commands during the image build process. Each RUN instruction creates a new layer on top of the previous one, and any changes made in one layer are preserved in subsequent layers. RUN instructions are typically used to install packages, update configurations, and perform other tasks required to prepare the image for use.

2. CMD: The CMD instruction is used to specify the default command to be executed when a container is started from the image. The CMD instruction is optional, but if it's not specified, Docker will use the default command defined in the base image. The CMD instruction can be overridden by passing a command to the docker run command when starting the container.

🔹How Will you reduce the size of the Docker image?

To reduce the size of a Docker image, I would follow these best practices:

1. Use multi-stage builds to create multiple layers in a single Dockerfile and copy only the necessary files and dependencies from the build stages into the final image.

2. Minimize the number of layers in the Docker image by combining multiple RUN instructions into a single layer and using && to run multiple commands in a single layer.

3. Remove unnecessary files and directories from the Docker image, including temporary build artifacts, log files, and cache files.

4. Use Alpine base images, which are lightweight and specifically designed for containerization, to significantly reduce the size of the final Docker image.

5. Compress and optimize files, such as images and assets, to reduce their size and the size of the final Docker image.

🔹Why and when to use Docker?

Suppose we need requirements to set up an end-to-end application including different technologies like a web server using node js and a database such as mongo dB and messaging system or an orchestration tool like ansible. We have all these issues for developing this application with all these components.

1. We have to ensure that all these components are compatible with the OS.

2. Suppose one service required one version and another service required another version.

3. We have to upgrade to new versions of these components or change the database.

4. One developer may be comfortable with one OS and another may be another OS.

5. We couldn’t guarantee that the application that we were building would run the same way in different environments.

Using Docker we can package all the dependencies in a Docker image and run this image to create a container instance. This Docker image can be run on any host machine. To overcome these issues we should use Docker. Docker makes application platform independent.

🔹Explain the Docker components and how they interact with each other.

Docker components are:

1. Docker Engine: The core component that allows users to create and manage containers.

2. Docker Registry: A central repository that stores Docker images for sharing and distribution.

3. Docker CLI: The command-line interface used to interact with Docker.

🔹Explain the terminology: Docker Compose, Docker File, Docker Image, Docker Container?

• Docker Compose: this is a tool for defining and running multi-container Docker applications.

• Dockerfile: a text file containing instructions for building a Docker image Docker.

• Image: a lightweight, standalone, and executable package that contains everything needed to run a piece of software.

• Docker Container: a running instance of a Docker image, isolated from the host system and other containers.

🔹What is ENTRY POINT in Docker?

In Docker, an entry point is a configuration option that allows you to specify the command that should be executed when a container is started from a Docker image. It is defined in the Dockerfile using the ENTRYPOINT instruction can be used to specify a default command that will be executed every time the container is started.

🔹What are the docker commands for the following:

- view running containers

- command to run the container under a specific name

- command to export a docker

- command to import an already existing docker image

- commands to delete a container

- command to remove all stopped containers, unused networks, build caches, and dangling images?

Here are the Docker commands for the operations you requested:

1. View running containers:

docker ps

This command will list all running containers.

1. Run the container under a specific name:

docker run --name <container-name> <image-name>

The docker run command is used to create and start a container based on a Docker image.

1. Command to export a docker:

docker save -o <file path> <image>

The docker saves command to save an image to a tar archive.

1. Command to import docker an already existing docker image

docker load -i <file path>

The docker load command loads an image from a tar archive.

1. Command to delete a container

docker delete <container id/name>

The docker delete command deletes a container.

1. Command to remove all stopped containers, unused networks, build caches, and dangling images

docker system prune -a

This command deletes all the topped containers, unused networks, build caches, and dangling images.

📍Conclusion

Docker has become an essential tool for developers and organizations to build, deploy, and run applications in a containerized environment. As the demand for skilled Docker developers continues to grow, it is crucial to be well-prepared for Docker-related interviews. In this blog, we have covered some common Docker interview questions, including the basics of Docker, Dockerfile, Docker Compose, and how Docker components interact with each other. Understanding these concepts and best practices can help you prepare for your next Docker interview and succeed in your Docker-related role.