Self-hosted runners with Github is a whole world of pain because it literally just runs commands on the host and does not handle provisioning/cleanup, meaning you need to make sure your `docker run` commands don't leave any leftover state that can mess up future/concurrent builds. It doesn't even handle concurrency by itself, so you have to run multiple instances of the runner.

Github supports ephemeral runners which are limited to a single job.

You can use `workflow_job` webhook to be notified of a new job, after that you need to call `generate-jitconfig` API to get a just-in-time configuration token, and then you can start a Github runner in ephemeral mode with the JIT token.

This allows you to orchestrate Docker containers, KVM instances, etc., which are used for a single time, and then destroyed.

There are some open source projects, such as using ephemeral Kubernetes pods with the ephemeral runners.

That was included in my "but then you have to self-host" :-).

As I said, I really like the SourceHut CI.