How to run Docker in Docker? A Complete Guide

Docker in Docker has revolutionised the way developers build, ship, and run applications. Docker Containers offer a lightweight and portable solution, allowing for easy deployment across various environments. However, there are situations where you might need to run Docker within another Docker Container, commonly referred to as Docker in Docker (DinD).

Imagine you are working on a project that requires multiple microservices, each with its own set of dependencies and configurations. In such cases, using Docker Hub allows you to create isolated environments for each microservice, ensuring that changes or updates to one do not impact the others. It facilitates easy testing and debugging, enabling developers to fine-tune their applications without disrupting the entire system.

In this blog we will learn how to effortlessly run Docker within Docker, empowering your containerised application like never before. We will explore the best practices and methods to achieve this while considering the advantages and disadvantages of such an approach and What is Docker all about. 

Table of Contents  

1) What is Docker?

2) Why would you run Docker in Docker?

3) Methods to run Docker in Docker  

      a) Using Docker-in-Docker (DinD) Image

      b) Using Docker-Compose

      c) Using Docker Socket  

4) Conclusion

What is Docker? 

Docker Hub is an open-source platform that empowers developers to automate the distribution of applications inside lightweight containers. These containers package everything needed to run an application, including code, runtime, libraries, and dependencies. Docker in Docker, on the other hand, refers to the concept of running Docker commands and containers from within a Docker Container itself. 

It has emerged as a game-changer in the world of software development and deployment. It is an open-source platform that supports developers to formulate their applications and all their dependencies into containers. These containers are portable, lightweight, and can run consistently across various environments, ensuring that what works on a developer's machine will also work in production. Docker has streamlined the process of building, shipping, and running applications, bringing unprecedented ease and efficiency to the software development lifecycle. 
 

Why would you run Docker in Docker?

Running Docker within Docker provides developers with increased flexibility to leverage the tool for tasks that might be challenging through other methods. Discussed below are two reasons justifying the same.

You can deploy a CI/CD Pipeline

Within certain DevOps environment, the Continuous Integration/Continuous Delivery (CI/CD) pipeline uses Jenkins or GitLab running in a container. This means that all instructions within the pipeline stages are carried out on the agent. If a Docker command is included among these instructions, it becomes necessary to execute it from within a container.

Creating a sandbox environment becomes a lot easier

A common use case for running Docker in Docker is using the containerisation platform as a sandbox environment. This allows for the isolation of Docker from the host environment. The environment can be destroyed easily by removing the container.

Interested to gain deeper knowledge about docker, refer to our blog on "Podman vs Docker"

Methods to run Docker in Docker 

Learn to set up Docker in Docker with our comprehensive blog. Includes step-by-step instructions, best practices, and how to integrate Docker Compose for streamlined container management. Read more to find out! Each method comes with its own advantages and considerations, making it essential for developers to choose from the most suitable approach based on their specific use case. These are several methods to run Docker in Docker. Let's explore three popular methods: 

Using Docker-in-Docker (DinD) Image 

One straightforward approach is to use a pre-built Docker-in-Docker (DinD) image. This method provides a nested Docker environment within the container. You can pull the DinD image and run it as a new container, effectively giving you access to another Docker daemon. 

The Docker-in-Docker (DinD) image is essentially a Docker Hub image that contains the Docker engine itself. By using this image, you can effectively create a nested Docker environment within a parent Docker container. This means that you have access to all the familiar Docker commands, and you can run and manage containers just as if you were using Docker on the host machine. 

For example, to run DinD, we can use the following command:
 

By understanding the appropriate use cases and best practices, developers can leverage Docker-in-Docker effectively to enhance their development workflows and build robust applications.  

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Using Docker-Compose 

Docker Compose is another excellent tool to run Docker in Docker. With Compose, you can define multi-container applications using a YAML file. By creating a Docker Compose file with a service that runs the Docker daemon, you can spin up Docker within a Docker container easily. 

The key benefit of Docker-Compose lies in its ability to define a multi-service architecture in a declarative manner. This means that you can specify the services and their configurations in the YAML file, and Docker-Compose takes care of orchestrating the containers based on that configuration. 

For example, you are developing a web application that consists of a frontend service and a backend service, and both services require specific dependencies and configurations. Instead of manually managing multiple docker run commands, you can define the services in the docker-compose.yml file, as shown below:
 

Developers can streamline their development workflows, ensure consistency across different environments, and build scalable and robust applications with ease using Docker-Compose.  

Using Docker Socket  

Using the Docker Socket is a powerful and efficient method to run Docker within Docker containers. Instead of creating a nested Docker environment like Docker-in-Docker (DinD), this approach leverages the host machine's Docker daemon, allowing containers to interact directly with the host's Docker engine.   

This method is particularly useful when you want to utilise Docker Hub functionalities without the overhead of running a separate Docker daemon within the container. For instance, to mount the Docker Socket, use the following command when running your container: 
 

Using Docker socket opens various possibilities and benefits such as: 

Sharing Docker Resources: Containers with Docker Socket access can use the host machine's Docker resources, such as networks, volumes, and images. This can significantly reduce resource overhead and storage requirements for running Containers.  

Streamlining CI/CD Pipelines: In CI/CD workflows, where isolated Docker environments are required for each build or test job, using a Docker Socket can enhance performance and reduce duplication of images, as each job can share the same Docker engine.  

Accessing Docker APIs: Containers with Docker Socket access can interact with the Docker API, enabling more advanced functionalities and integration with other tools.
 

Conclusion 

Docker in Docker can be a powerful tool when used correctly for testing and development. By following best practices, you can mitigate potential issues and enjoy the benefits of isolated Docker environments. Remember to be cautious when using Docker in Docker in production, as it may pose security risks and resource challenges. As Docker continues to revolutionize the world of software development, Docker in Docker adds another layer of versatility and flexibility to an already innovative platform, allowing developers to tackle complex challenges with confidence and efficiency. 

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