Activate GitLab-CI

Goal

Figure out the minimal requirements for activating CI. Only one job for now to see how everything behaves.

Checklist

• Decide single initial job :
  • meson build, no checks
  • on a single distribution
    • @alatiera which distro ? -- fedora
• Verify/implement minimal changes:
  • gstreamer/gst-build>
    • build scripts are present
    • Anything else ?
  • gstreamer/gst-ci>
    • Script to generate manifest gst-ci!2 (merged)
    • .gitlab-ci.yml to be included in all other code modules gst-ci!3 (merged)
    • Merge the update of Dockerfiles (gst-ci!1 (merged)) - we could trim it down, but I do not think it is necessary, even though we won't use all of it as a start.
    • add the gitlab-ci template to be included in the "other modules on which we want to activate gitlab-ci"
  • other modules on which we want to activate gitlab-ci
    • Add gitlab-ci.yml which imports/includes the template