Document how to set up Gitlab CI NVIDIA GPU bound builds

**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](https://github.com/game-ci/documentation/files/9397676/GameCI.GPU.Setup.md)