Document how to set up Gitlab CI NVIDIA GPU bound builds

[JpGallegos]

Context

I managed to get Unity builds to run on NVIDIA GPUs for Gitlab CI pipelines. I would like to add this to the shared knowledge base of this community. Most of the work consists of setting up the build machine and gitlab-runners. This was done on computers running Ubuntu 20.04 with gitlab-runners using privileged docker executors.

Additional details

Attached is screenshot proof of the results and rough documentation of how I managed to achieve this.

GameCI GPU Setup.md

[webbertakken]

Amazing pioneering! And thank you so much for documenting the process.

In GitHub (Cloud Runner) documentation we already have a section called Advanced topics.
Please feel free to create such a section under GitLab as well and add this first topic.

It would be helpful if you could brush up the documentation as much as possible where needed, so that it can make for a solid basis on this topic. PRs welcome!

[JpGallegos]

I think there is some of the steps that can be integrated into https://gitlab.com/game-ci/unity3d-gitlab-ci-example. Not sure if that is something that you guys would be interested in.

[webbertakken]

Definitely! PRs are also welcome in the example repositories.

I do think it is important that it doesn't break the current flow, and is treated as an opt-in feature based on some parameters.
For example, -nographics can't be removed by default (probably not even if there are NVIDIA_VISIBLE_DEVICES, but on an "as needed" basis) because it has been causing many issues - differently between editor versions.

I also like that you kept comments with references to issues, inside the markdown file you linked. These can be very helpful.