Debian's Salsa team has their own registry of images they maintain, so
use them instead of ones maintained by gitlab.
At https://salsa.debian.org/salsa-ci-team/pipeline/container_registry
one can view the various images they maintain.
Switch to using 'unstable' instead of 'sid'. The idea is to later also
add steps based on 'stable' and then 'unstable' matches better.
It's common to define variables/constants in the top of a file, so do
that here too.
Move 'stage' field directly under step name as it's quite significant
and seen in other salsa-ci.yml as well.
Move 'image' line after 'stage' line as it's quite important as well.
Diederik de Haas