builds.sr.ht can use private repos on git.sr.ht and elsewhere, but you have to configure each job with an SSH key that has access to your account.
The resulting build manifest should look something like this:
image: # ...
secrets:
- c043e12e-a297-4ece-b09c-bb5a99dc4094
sources:
- git@git.sr.ht:~yourname/your-private-repo
commit bf6aab980baaf2357fe355e3dfdadec371d6a014 Author: Conrad Hoffmann <ch@bitfehler.net> Date: 2023-04-19T21:51:23+02:00 packages: update Alpine version