Add gitlab CI recipe to build app using flatpak-builder

This recipe should be used in Mildenhall projects to automatically build the flatpak bundle.
This install the runtime and SDK from https://images.apertis.org/public/flatpak/mildenhall, then build the bundle using flatpak-recipe.yaml found at project's root.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com