Resolve "CI Builds auf Untrusted Runner"

Acceptance criteria

• Coverage is higher or equal to coverage on master
• CI runs without problems
• Linter does not contain any additional issues
• Code doesn't contain TODOs, unused functions or console outputs
• Code has been inspected and deemed to have decent quality
• Code does contain license headers
• If necessary, the changes are documented in the changelog
• If this is a bug fix, a test covering the bug is present [effectiveGo]: https://golang.org/doc/effective_go.html

Closes #282 (closed) , Closes #292 (closed)

This MR improves the CI by:

• merging build and test stage to only the test stage to reduce build time (by 2 minutes)
• add untrusted docker build job to build image on generic runners

Todo:

• fix Version number for Branches
• cleanup
• doku of pipeline