[CI] Make sure all pacxx-repositories follow a reasonably consistent .gitlab-ci.yml style

Depends:

• #20 (closed)
• #21 (closed)

Repositories:

• https://gitlab.dune-project.org/joe/pacxx-docker (!61 (merged), !62 (merged), !63 (merged))
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-runtime (pacxx-runtime!35)
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-samples (deferred to pacxx-samples#12)
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-llvm2 (pacxx-llvm2!6)
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-rv2 (pacxx-rv2!5)
• https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-clang (pacxx-clang!3)

Replaces: #22 (closed)