e04f14eb/dc9dc70f introduced preprocessor warnings to deny the use of
any `alsa-lib` older than `1.2.1`, and with a future patch disallowing
warnings entirely through `-Werror` we now need a distribution that
serves a new enough `alsa-lib`.
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/653>
This updates things based on changes in the templates that we use. Also
pins the ref in the template repo so that our build does not break when
the template parameters change.
This does mean that we should likely periodically check the ci-templates
repo, but this seems to be better than the build breaking unexpectedly.
This adds a Dockerfile to generate a Docker image with the required
dependencies on top of the standard Ubuntu 18.04 image. The Gitlab CI
then runs the PulseAudio build within this image.
