Add minimal CI

This just do a build on latest Fedora, .gitlab-ci.yml-ignored contains
much more that could be backported later. This is better than nothing.

It uses freedesktop ci-templates for managing docker images:
https://freedesktop.pages.freedesktop.org/ci-templates/templates.html
2 jobs for master in 4 minutes and 40 seconds (queued for 2 seconds)