Add support for GPU-acceleration

[cloudymax]

[Draft, more changes inbound]

This PR adds GPU acceleration to the Unity Builder action by providing an addition entrypoint script which:

• Enables X sessions from non-root users
• Installs user-space GPU drivers
• Creates a headless Xorg configuration
• Runs a desktop session (Xorg + Xfce or IceWM)
• Runs a remote desktop utility (VNC + noVNC)
• Runs a desktop video capture session (ffmpeg)

These changes work in combinations with game-ci/documentation#419 as a solution for the following issues:

• Docker GPU Support? unity-test-runner#102
• NVIDIA Support unity-test-runner#113
• Document how to set up Gitlab CI NVIDIA GPU bound builds documentation#323
• No docs for a standalone Docker environment documentation#386

[github-actions]

[codecov-commenter]

Codecov Report

Merging #577 (fcc68d4) into main (a13443a) will not change coverage.
The diff coverage is n/a.

❗ Current head fcc68d4 differs from pull request most recent head 3771485. Consider uploading reports for the commit 3771485 to get more accurate results

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #577   +/-   ##
=======================================
  Coverage   37.02%   37.02%           
=======================================
  Files          77       77           
  Lines        3041     3041           
  Branches      632      632           
=======================================
  Hits         1126     1126           
  Misses       1765     1765           
  Partials      150      150           

Files Changed	Coverage Δ
src/model/docker.ts	10.63% <ø> (ø)