Remove macOS specific code / support + shellcheck should avoid python, regardless of permissions

[NorseGaud]

Description

Supporting macOS is a nightmare. Users can run a headless linux VM, mount the docker-mailserver directory into it, and run make commands if they are on macOS.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• [ https://github.com/[BUG] Building Dockerfile locally fails #2023 ] New and existing unit tests pass locally with my changes

[NorseGaud]

I'm sort of stuck running the tests and stuff locally... Per #2467, postfix won't install due to the builder container not having a full FQDN

[NorseGaud]

I made two unrelated to the PR, but blocker fix changes in this :

1. Avoid leaving build container around (they would pile up while testing the build)
2. Using grep to avoid python script since my virtual machine mount forces all of the target/bin/ files to be executable and fails shellcheck.

[casperklein]

LGTM. But there are still some ${@:+$@} left in setup.sh and lint.sh

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: 45068ff

[georglauterbach]

I'm currently on vacation in Madeira, therefore only had a quick look, but LGTM