I think Drone CI is at the moment best self-hosted CI service. It’s fully FOSS, available both as service, runners or both. Both service and runners can be hosted either in cloud (with some free tier for Open Source projects) or self-hosted. Finally, contrarily to Gitlab CI or Github Actions, it’s not tied to any specific Git service vendor (although I read somewhere that Gitlab CI can be painstakingly made to work with other Git providers with some workarounds). Also there are no special paid-tiers, all functionality is available for self-hosted versions.
My favourite combination for self-hosted git repos + CI is Gitea + Drone as both are fully FOSS and easy to self-host. Hope it doesn’t come out as a form of unwanted advertisement. If so, please delete this post. I’m not connected with those projects in any way besides using them. I was just looking for this kind (completely FOSS, with possibility to self-host either in parts or entirely) solution and for long couldn’t find alternatives with features comparable to Github/Gitlab + their CIs. I just wanted to share this findings with Julia community, since from what I’ve seen packages ecosystem heavily depends on GitHub & Travis-CI combination.