gitlab-ci: Create separate docker images for Debian stretch & buster

The main objective of this MR is to make the docker image creation process simpler and less fragile, mostly by creating separate images for Debian stretch & buster instead of a single hybrid image.

There's also some simplification and re-balancing of jobs, to avoid adding more than one (inevitable) additional build+test job. As a bonus, both meson & scons are now tested against all supported LLVM releases, while keeping the duration of a single build+test job within around 10 minutes.