ci: add support for GitLab CI

ci: add support for GitLab CI

We already support Azure Pipelines and GitHub Workflows in the Git
project, but until now we do not have support for GitLab CI. While it is
arguably not in the interest of the Git project to maintain a ton of
different CI platforms, GitLab has recently ramped up its efforts and
tries to contribute to the Git project more regularly.

Part of a problem we hit at GitLab rather frequently is that our own,
custom CI setup we have is so different to the setup that the Git
project has. More esoteric jobs like "linux-TEST-vars" that also set a
couple of environment variables do not exist in GitLab's custom CI
setup, and maintaining them to keep up with what Git does feels like
wasted time. The result is that we regularly send patch series upstream
that fail to compile or pass tests in GitHub Workflows. We would thus
like to integrate the GitLab CI configuration into the Git project to
help us send better patch series upstream and thus reduce overhead for
the maintainer. Results of these pipeline runs will be made available
(at least) in GitLab's mirror of the Git project at [1].

This commit introduces the integration into our regular CI scripts so
that most of the setup continues to be shared across all of the CI
solutions. Note that as the builds on GitLab CI run as unprivileged
user, we need to pull in both sudo and shadow packages to our Alpine
based job to set this up.

[1]: https://gitlab.com/gitlab-org/git

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>