Merge branch 'ps/ci-gitlab'

Add support for GitLab CI. * ps/ci-gitlab: ci: add support for GitLab CI ci: install test dependencies for linux-musl ci: squelch warnings when testing with unusable Git repo ci: unify setup of some environment variables ci: split out logic to set up failed test artifacts ci: group installation of Docker dependencies ci: make grouping setup more generic ci: reorder definitions for grouping functions