I found a couple of issues with cache and artifacts in current experimental (9.1.1). Digging further they happen to be upstream bugs.
A couple of cache/artifacts-related bugs have bee, fixed in 9.2 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/commit/adfc38756f38a2114a402a799c46da85a542419b (I can't get them working with the Debian's version) That would open the path to do ci testing before uploading to gitlab (not only job per job but test possible jobs' dependencies issues) Upgrading Debian's package would be highly appreciated