gitlab CI: update to use the fdo ci templates

Instead of building a new debian image every time we build the repo, let's build
it once and re-use it. This way we're more  likely to spot actual bugs vs issues
with the distribution (or mirrors). This is the same approach mesa, gstreamer,
libinput, etc. already use.

The pipeline consists of two parts: container-build and distribution-image. Both
use the FDO_DISTRIBUTION_... variables to do their thing, the former to build an
image, the latter to use that image. The container-build step is a noop if an
image with that tag already exists in the registry, the templates take care of
all that.

The .debian.buster section groups the variables needed to easily extend all jobs
requiring buster. Not all variables are used by all jobs but meh.

The grouping is slightly odd because some debian-specific variables are in
the global variables and others in the .debian.buster section. This grouping
will make things easier if we extend to build on other distributions - then we
have all packages and tags in one place.

Because buster doesn't have a recent-enough version of meson, we install that
from pip.

Fixes #79

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>