This creates a container that can be used to build FreeBSD binaries natively on Linux
(this finishes by running 'docker build -t freebsd-cross-build .')
docker run --rm -it freebsd-cross-build
There is /freebsd/bin on the path. It has all of the build tools (e.g. x86_64-pc-freebsd9-gcc)
It is likely you would add a -v switch to the run (to put your code on a mount).