Overhaul container entrypoint script and Docker docs

[twz123]

Description

Add some extra smarts to the k0s container entrypoint script. By adding cgroup setup capabilities, it's no longer necessary to use the host's cgroups when running k0s in a container. Track down and document the necessary security-related container flags, so there's a slightly more audited alternative to just using --privileged. Re-add the /run directory as tmpfs, as the data in it is not meant to be persistent and could potentially cause problems when containers are restarted.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• 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
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings