In April 2018 we started to experiment more seriously with GitLab and GitLab CI.
GitLab CI
Support for GitLab CI was added in https://trac.sagemath.org/ticket/24655.
Free Runners with Google Compute Engine
Currently, Google gives everybody who signs up $300 of credit for 12 months. They won't charge you unless you explicitly upgrade, and there are a few limitations (e.g, at most 8 CPUs per regions.) Btw., they don't charge you if you upgrade unless you run out of the free credit.