Document that the image version must match GODOT_VERSION

[benwest]

I don't know much about Docker so it wasn't obvious to me that I need to change both the GODOT_VERSION env var and the version tag on the image to match. Otherwise the workflow fails with mv: cannot stat '/root/.local/share/godot/export_templates/4.6.stable'.

I thought the env var would get passed to the ARG I saw in the Dockerfile but I now understand that's a build time option. Would have been nice if this was documented for dum dums like me.

[Calinou]

I'm wondering if we can get rid of the GODOT_VERSION requirement by using a wildcard for the folder name. There is only going to be a single folder within .local/share/godot/export_templates/ in this CI use case anyway.