I left Github a while ago and have been relying on simple pre-push scripts in my workflow, but would like to be able to test PRs from others without putting my machine at risk. Besides codeberg and radicle (neither of which have reliable CI), I also have a build machine, where I could run CI jobs, however it is important that the CI jobs can also run locally so that external people do not require access to the build machine.
Is there a CI that can do those things (run locally and remotely)?
Run your CI in a sandbox.
For example gitlab allows you to run in a docker image.
Unless the attacker knows a docker CVE or is willing to waste a specter style 0-day on you, the most they can do is waste your cpu cycles.
Yep. Hell, be very paranoid and run it in a container on a runner VM on your box if you like.
And you can use podman or sysbox there.