Add Docker CI/CD for platforms, versions & build tools.

[AlexDarigan]

There are four different build tools for Mono (JetBrains MSBuild, VS Msbuild, Mono, .NET cli). A fifth if you count no-mono
There are three major platforms: Windows, Mac & Linux (default to Ubuntu as it is the most ubiquitous).
3.4 & 3.5 are the significant versions of Godot. Don't think there is enough on 3.3 to consider continue. Anything lower is pointless.

Therefore 5 * 3 * 2 = 30 different CIs (is this right?)
See if we can try to aim for combined actions per platform (so only 3 major with 15 * 2 variations)

[AlexDarigan]

In retrospect, we only need a single build tool since we're invoking it externally from Godot.