If you want an alternative to GitHub Actions, you could self-host Forgejo Actions, but I'm not that happy with the design.

I much prefer Woodpecker CI, which is an open source fork of Drone.io. It supports multiple Git backends like GitHub, Gitea, Forgejo, Gitlab, Bitbucket. It supports running jobs locally, on Docker, and on Kubernetes. And there's autoscalers built in for AWS, Hetzner, Linode, Vultr, and Scaleway. There's a bunch of 3rd party plugins (https://woodpecker-ci.org/plugins) for custom integrations. The UX is also very simple, with OAuth used not only for authentication/authorization but also setting up & accessing repos. The system architecture is great, with separate components that run stateless connected to a database, and a custom plugin is any program that takes environment variables and does stdio. The config file is a good balance of ugly YAML and convenience syntax like shell-style parameter expansion variables.

It probably takes less than 15 minutes to install, set up, and run WoodpeckerCI for a small team, so it's not a big investment to try out or host. With the autoscaling plugins it lets you scale your workload up to whatever size. Honestly you could run it on a laptop since it's written Go.

(to clarify for beginners: the config file docs are found in a section called "workflow syntax" (https://woodpecker-ci.org/docs/usage/workflow-syntax) and variable parameter expansion is buried deep in an environment variables page called "string operations" (https://woodpecker-ci.org/docs/usage/environment#string-oper...). poorly organized docs aside, the system itself works well)