Factor out reusable CI logic into vendor-neutral source code

[sanmai-NL]

Currently you have GitLab CI as well as a GitHub Actions declarations. It would be better to factor out e.g., shell scripting incorporated in the related YAML files into dedicated scripts. These could then be called and reused (composed) from multiple contexts. This change would improve maintainability (DRY). Furthermore, such scripts can also be run locally and used for container image production etc..

Side note: I advise to not go the way of shell scripting, since it looks simple in the beginning but then gets out of hand easily. Shell scripts also harder to port to Windows or various UNIX-like OS'es due to subtle differences, and at some point in some context these cases will arise ... I advise to write Python scripts that use the standard library exclusively.