Document how to perform a stage2 build locally inside the VMR

[ViktorHofer]

[ViktorHofer]

@MichaelSimons IIRC you shared instructions ~a year ago how to do this. Can you please share them again or ideally document them?

[ViktorHofer]

OK this works, i.e. in a codespace:

cd dotnet
./prep-source-build.sh && ./build.sh -sb
mkdir ../current-sdk
tar -xzf artifacts/assets/Release/dotnet-sdk-10.0.100-preview.2.25110.1-centos.9-x64.tar.gz -C ../current-sdk
mkdir ../current-packages
tar -xzf artifacts/assets/Release/Private.SourceBuilt.Artifacts.10.0.100-preview.2.25110.1.centos.9-x64.tar.gz -C ../current-packages

git clean -xdf
./build.sh -sb --with-sdk ../current-sdk --with-packages ../current-packages

[MichaelSimons]

This is a duplicate of #4422