I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

Looks great, thanks!

Side note, (inspired by how CI does it to keep logs slightly tidier and comparable) I personally mount the volume with fixed path at root of container's FS:

- docker run --rm --volume $(pwd):$(pwd) --workdir $(pwd) --env ROOTFS_DIR=/crossrootfs/x64 -ti  $TAG ./build.sh -cross -os FreeBSD
+ docker run --rm --volume $(pwd):/runtime --env ROOTFS_DIR=/crossrootfs/x64 -ti "$TAG" \
+   /runtime/build.sh -cross -os FreeBSD

@am11 I do something similar for that too as it keeps things cleaner for runs :)

I decided to leave it for now, if the syntax ends up being confusing or more people raise "build not working with docker" questions then I will change that to be a bit more clear.

BTW it seems like we need python to build. I just bump to this while finally getting my build working again on fresh system

Hmm. actually building is here: https://github.com/dotnet/runtime/blob/main/docs/workflow/building/libraries/freebsd-instructions.md

May need some updates as well. It may be better to point at the cross-building scripts instead of my old binary snapshot.

Those build instructions should be updated at some point. They also seem to duplicate the building instructions under https://github.com/dotnet/runtime/blob/main/docs/workflow/requirements/freebsd-requirements.md

How should they be reworked/changed? Change docs/workflow/building/libraries/freebsd-instructions.md to point to docs/workflow/requirements/freebsd-requirements.md? This would be the easiest/laziest(?) way as the later includes building instructions using both Linux docker and native FreeBSD toolchain.

The docker instructions are fine for now but if I am updating the doc more I will update the container referenced and fix up the command as per @am11 's suggestion.

Native FreeBSD toolchain will need a dependency addition of python3 which both net6 and net7 now need. Currently, using a community built net6 SDK as a bootstrap, it will build the current dotnet/runtime/HEAD with only 4 lines of changes to /eng/Versions.props. These will need to be added too.

I am fine with any of the above but also open to other ideas

Yes, I'm on similar path. And https://lists.freebsd.org/pipermail/freebsd-current/2020-May/076043.html
Had to add it from ports.

I think freebsd-requirements.md should have instructions how to install/run runtime. The building document can reference it or duplicate the package for convenience. I like the one where you can select & paste commands to the shell and that makes it work.