fix(builder): compile Go binaries with cross-compile support

With the root builder Dockerfile, we were simply compiling and building
binaries that were compatible with our docker image. This had the
disadvantage for forcing us to use anything in the current context.
For example if we relied on a package higher up in our tree, it would
not be sent to Docker's build context, making us run loopholes just
to compile a binary. Go to the rescue!

Go has cross-compilation support, so if you compiled Go from source on
Windows but wanted to be able to compile binaries for linux, you can.
This moves the compilation onto the host instead of inside a Docker
container, which will allow us to use the full context of our local
GOPATHs to compile our binaries before injecting them into the final
image.