Make it possible to run init/systemd inside the build container

I would like to start the build container with a working systemd inside. This means having /sbin/init as PID 1.

Currently there is no good way to do this as the container command is hardcoded:
https://github.com/buildkite/agent-stack-k8s/blob/2bf25843d9aaa1cbf2b0b0bb6b742cb1187b5e1c/internal/controller/scheduler/scheduler.go#L248

It would be nice to be able to override this command to be able to start /sbin/init and then the buildkite-agent with its arguments.

We could of course hack this into a container image derived from the official one, but would rather not.

Any help making this work would be highly appreciated.