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.
That was included in my "but then you have to self-host" :-).
As I said, I really like the SourceHut CI.
Forgejo handles all of this quite well and is, in my experience, muuuch faster than GHA
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.