I've written up my adventures from earlier today working out how to cross-compile a @rustlang project for FreeBSD using docker (without emulation).
https://www.wezm.net/technical/2019/03/cross-compile-freebsd-rust-binary-with-docker/ …
Oh wow, I've used cross before for embedded work. I didn't realise it supported alternate OSes too. I'll investigate and probably add a note to my post. Thanks for pointing it out.