Nspawn – Create Containers From Docker Images

You’ve already seen that Systemd has an entire container engine under the hood.
You may created a fully functional Debian container in it.

But what can you do when you need a different distribution?

You can convert a Docker image to an Nspawn container.

Objective

1. Use podman as the container engine to manage the Docker images.
2. Create suitable containers for Nspawn.
3. Use Btrfs for Nspawn templates.

In the example I will use a Rocky Linux image.
You can choose any other Linux image from your trusted Docker source.

Rocky Linux Image Example

Pull the official Rocky Linux image from the Docker Hub with podman.

podman pull docker.io/rockylinux/rockylinux:10

Start a container from the downloaded image.

podman run --name rocky-template -it docker.io/rockylinux/rockylinux:10 /bin/bash

Install the necessary tools and set up the system.

dnf update -y
dnf install -y dbus NetworkManager
systemctl enable NetworkManager
systemctl enable dbus
exit

Now you can export the container as a tar archive.

podman export rocky-template -o rocky-10-template.tar

Create a Btrfs subvolume for this template and move the base there.

sudo btrfs subvolume create /var/lib/machines/rocky-10-template
sudo tar -xf rocky-10-template.tar -C /var/lib/machines/rocky-10-template

Remove the machine-id and hostname from the Nspawn template.

sudo rm -f /var/lib/machines/rocky-10-template/etc/machine-id
sudo rm -f /var/lib/machines/rocky-10-template/etc/hostname

Make the template image read-only:

sudo machinectl read-only rocky-10-template yes

You can clone it and run a container from it.

sudo machinectl clone rocky-10-template new-container

Final Thoughts

As you’ve seen you can convert many Docker images to functional Nspawn containers.
Images based on complete Linux distributions are particularly well suited for this approach.

This way, instead of application containers you can run OS containers easily.
The process is repeatable and easy to automate.