Add Gitlab-CI definition file that can test each commit to this repository

Modeled after what Debian Salsa CI team uses, see
https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/salsa-ci.yml

Modified a bit to stay under the 4 MB log size limit
and 100 MB artefact limit.

Attempted to get all of Salsa CI working, but ran into failures and was
unable to overcome all of them, so leaving some parts of the pipeline
disabled for now.