Add PredefinedShell configuration

What does this MR do?

The key idea of this change is the build.go change. This logic decides wether we use helper image or not (aka Predefined in the code).

So we introduce a PredefinedShell configuration, which allows to specify a different helper for the helper image compared to the user script.

Eventually it may make sense to allow the user to choose bw shell and powershell inside the gitlab-ci.yml (or maybe autodetect from the dockerfile shell configuration)

Why was this MR needed?

be able to use bash on windows containers and powershell on linux containers

Are there points in the code the reviewer needs to double check?

I am totally new to this code, and also on golang, so please be patient with me.

This MR is here to introduce discussion on the code. I am willing to drop everything and restart according to advices from gitlab-runner maintainers.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

#4725 #4021 (closed)