Docker will only let you cross-compile to OSes with the same kernel, which is relatively limited. A full set of say vagrant files would be slower, but a lot more polyvalent (but even then it would not let you, say, compile a windows binary from linux).
You can cross-compile to completely different environments. It just can be tricky to set up, so prepared docker images with e.g. the toolchain to build a Windows executable on Linux are great to have, and what is provided here.
I think it's a nice idea in theory to provide cross-compilation support by means of emulating all possible target architectures on the compiling machine.
Then I remember that I'd be managing a custom heterogenous compute cluster just to compile some code.
It's not emulation, it's cross-compilation which has existed basically forever. You don't have to target the same instruction set as the one you're compiling on.
Go doesn't really use libc all that much. Syscalls are also implemented in pure go + as, so there aren't really header files to provide. That makes for a great cross-compilation story.
