I again spend two days of digging into (self-hosted) CI.
It is shocking how software development is dependent on #GitHub, their action runners and hosting of libraries!
I try to avoid it. For most projects I use #CodeBerg. Codeberg CI-support gets better and the public runners they offer are already quite good. The situation already has improved so much over the last two years.
I wanted reproduceable tests that also run in the CI, exactly the same way. In my case this requires the CI job to span up containers: docker-in-docker. This is not something Codeberg offers, so again I tried to configure my self-hosted forgejo-runner. The documentation is still a heavy read and there are no clear guidelines at all. I went from one error to the next for two days straight. In the end I gave up, I could not get directories from the job container mounted correctly in the containers started by the job container.
If anyone would offer help, I would take any advice.