A Powerful Technique: Modifying a Host System from Within a Docker Container

Use Containers to Manage Your Server’s Configuration

The ‘Ubuntu System Administration Guide’ introduces an advanced and powerful pattern used in modern infrastructure management, particularly with Kubernetes: using a container to modify the host system it’s running on. This allows you to package your configuration tools into a container and run them safely against your server.

How It Works: Volume Mounting

The key to this technique is mounting parts of the host’s filesystem into the container. The book provides a simple example where the host’s `/etc` directory is mounted into the container at `/mnt/etc`:

volumes:
  - /etc:/mnt/etc

From inside the container, you can now read and write files in `/mnt/etc`, which directly modifies the files in `/etc` on the host. This allows you to run a container with specific tools (like `sed`, `awk`, or a custom script) to perform a configuration update without having to permanently install those tools on the host server.

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This post is based from content of the book Ubuntu System adminstration guide. And the book can be found here https://www.amazon.com/stores/Mattias-Hemmingsson/author/B0FF5CQX13