Nested Docker Containers on Red Hat Linux: A Comprehensive Guide

Introduction:

Docker has revolutionized containerization, allowing applications to be isolated and deployed efficiently. But did you know that you can run Docker itself within a Docker container? This technique is called “nested Docker containers” and can be a valuable tool for specific use cases. In this blog, we will walk you through the process of launching a container on Docker, installing Docker within that container, and then launching a containerized Docker instance within Red Hat Linux.

Prerequisites:

Before we begin, ensure you have the following:

1. Red Hat Linux Installed: Make sure you have Red Hat Linux up and running. You can use any version supported by Docker.

Step 1: Launching the First Docker Container

To begin, open a terminal on your Red Hat Linux system and execute the following command to start a new Docker container:

docker run -it — name outer_container ubuntu:latest

In this command:
- `-it` allows you to interact with the container using an interactive terminal.
- ` — name outer_container` assigns a name to the container for easier reference.
- `ubuntu:latest` specifies the base image to use for the container. In this case, we use the latest version of the Ubuntu image.

Step 2: Installing Docker Within the Outer Container

Now that we have launched the outer container, let’s install Docker within it. This allows us to use Docker commands within the container itself. Perform the following steps:

a. Update the package index inside the outer container:

apt-get update

b. Install necessary packages to allow Docker installation:

apt-get install -y apt-transport-https ca-certificates curl software-properties-common

c. Add the official Docker GPG key:

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg — dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg

d. Add the Docker repository:

echo “deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable” | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

e. Update the package index again:

apt-get update

f. Install Docker:

apt-get install -y docker-ce docker-ce-cli containerd.io

Step 3: Launching a Containerized Docker Instance

Now that Docker is installed within the outer container, we can start a new Docker container inside it. This will be our “nested” Docker instance. Run the following command:

docker run -it — name inner_docker — privileged docker:latest

In this command:
- ` — privileged` is necessary to run Docker within Docker, as it requires certain permissions.

Step 4: Testing Nested Docker Containers

To verify that everything is working as expected, run the following command inside the nested Docker container:

docker info

This command will display information about the nested Docker environment, indicating that it is operational.

Conclusion:

Congratulations! You have successfully launched a container on Docker, installed Docker within that container, and then launched a containerized Docker instance inside Red Hat Linux. Nested Docker containers can be useful in various scenarios, such as testing Docker workflows within isolated environments. However, remember that nested containers come with some security implications, so use them judiciously and always follow best practices for container security. Happy containerizing!