Regression: Windows docker instructions leave out required registry settings for UWP

[st0nerhat]

Bug description

I updated the windows docker image instructions in PR #333. This involved removing instructions for adding Windows SDK registry changes. While these are not needed for IL2CPP builds, it turns out they are needed for UWP builds. See this discord thread.

Steps to reproduce

1. Configure a Docker windows container as described in https://game.ci/docs/docker/docker-images.
2. Setup an empty Unity project and bind mount it into the container.
3. Build the Unity project with -buildTarget WindowsStoreApps.

Observe this error:

Exception: ‘UnionMetadata\Facade\Windows.winmd’ not found, do you have Windows Universal Platform SDK installed?

Expected behavior

The game successfully builds.

Additional details

I plan to add these steps back in with additional hints on when and where to run the registry script.

[carl010010]

If it helps this was observed using Visual Studio 2019 and UnityCi windows-2020.3.30f1-universal-windows-platform-1.0.1