gitlab CI: use the 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.
Two parts here:
distribution-image. Both use the
FDO_DISTRIBUTION_... variables to do their thing, the former to build an image, the latter to re-use that image. The
.debian.buster job is there to have one point for the various distribution variables that we can easily
extend from, not all variables are used by all jobs but meh.
The grouping may look a bit weird because some debian stuff is in the global
variables: and other stuff is in the
.debian.buster section. This is primarily so that if we extend to build on other systems, we can have all packages and tags in one place. See the libinput one for reference.
Fixes #79 (closed)