Backport GitLab CI pipeline to v2020

Let v2020 to be built on GitLab CI, to make v2020 usable on downstreams switching to Azure.

Unlike for v2021dev2, this does not drop the Jenkins pipeline as we're on a stable release.

This required some minor change on the Jenkins pipeline as the sdk image recipes now expect a local devroot rather than an URL. A test run of the Jenkins pipeline pointing to this branch can be seen at https://jenkins.apertis.org/view/apertis-v2020/job/playground/job/em/job/debos-image-build/3/