CI: use the wayland-ci-templates to manage the images

libinput, xserver, xf86-video-amdgpu and mesa all have a similar-ish CI: they build a container to cache all of the dependencies and then reuse it.

When we started discussing about adding a similar CI stage to weston, it came clear that we would be rewriting yet an other time the same scripts. From this discussion, the repository https://gitlab.freedesktop.org/wayland/ci-templates/ started, and is aimed at providing such templates for our projects. This way, we have a central place were the CI can be tuned and we can host our own bootstrap image that create the other containers.

There shouldn't be a difference for users, but this way we do manage all of our containers from freedesktop.org

MR for the other projects:

• weston: wayland/weston!117 (merged)
• libinput: libinput/libinput!181 (merged)