Use CI templates from wayland/ci-templates

We have started an effort to unify the various similar CI scripts that are used across freedesktop.org.

Some projects (weston, xserver, libinput) are using a similar workflow: they create a base CI image and reuse it in a second stage. Mesa is using the same process, so it can benefit from the same templates.

Benefits are:

• no more docker-in-docker (a plain container is enough to build the image)
• fewer lines of codes
• a reduced image size (from 900MB to 700MB)*
• we share in common the templates, so if there are bugs, CVE, we can update them more easily
• simpler install script (instead of a Dockerfile)

For references, here are the various MR where the discussion happened:

• wayland/weston!98 (closed) (closed in favor of 117)
• wayland/weston!117 (merged)
• xorg/xserver!149 (merged)
• libinput/libinput!181 (merged)

From a mesa developer point of view, there should be no differences: the image is still tagged with UBUNTU_TAG, changing it will force a rebuild, and forks will attempt to grab the upstream image first, and will build it if not found.

* Few reasons for the reduced image size:

• the template takes care of not installing the man pages
• the template takes care of cleaning up large files not used after the installation
• the Dockerfile contained multiple RUN statement, each of which creates a new layer, with all of the temporary files mentioned above kept