ADD Gentoo support

What does this merge request do and why?

This merge request adds support for installing Gitlab on Gentoo linux, both through asdf and manually.

The procedure is similar to other distros, a specificity of Gentoo being that it allows several versions of ruby to coexist, similar to having a distro maintained and system wide equivalent of rvm. Added some instructions about that in the documentation for manual build, for Gentoo users who may not be familiar with how it handles ruby.

How to set up and validate locally

A docker image is provided to test Gitlab install on Gentoo. It provides both an image to test the asdf method of installation and one to test the manual method of installation. The asdf image is ran every month, through a scheduled pipeline. Sadly, the manual image is taking too long to build to run on Gitlab-CI (it runs for more than 3 hours), so it can't be automated. Instead, I'll run it locally and create issues on the repos so that you know it has been tested. Not ideal, obviously, if you know of a way to bypass the 3 hours limit, I'm all hear. :)

For the record, I've tested successfully both methods - the asdf one and the manual one - on two different gentoo systems. Congratulations to everyone, by the way : I've rarely seen a webapplication being so straightforward to install without docker, especially given the amount of services and complexity of the whole. It just worked™. Despite of that, if people need help installing on Gentoo, you can send them my way.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.