Draft: Resolve "Improve on gitlab ci/cd script"

Closes #935 (closed)

• define environments for staging (aka develop) similarly to dev
• checking the rules because some of them does not make sense anymore (like: $CI_COMMIT_REF_NAME =~ /^dev-.*$/)
• checking the way how initialisation of submodules works
• get rid of docker login waning (echo $CI_BUILD_TOKEN | docker login --username foo --password-stdin)
• cleanup (eg. using before_script more often)
• regularly rebuild the image from scratch to avoid issues like #937 (closed) (--no-cache)