golang: document how to make a "slim" cross

[tianon]

If someone only wants one or two platforms and don't feel like they need the full golang:cross image, they can use something simple like:

FROM golang
RUN cd /usr/src/go/src && GOOS=windows GOARCH=amd64 ./make.bash --no-clean
RUN cd /usr/src/go/src && GOOS=darwin GOARCH=amd64 ./make.bash --no-clean

[proppy]

/cc @jbeda

[jbeda]

I'll give this a try. It is really an interesting bandwidth vs. compute trade off -- download the pre-built stuff for all 20+ platforms or build the subset stuff locally. If you have fast pipes and slow local compute, you download. If you have slow pipes and fast local compute you build.

[tianon]

As part of this, let's be sure to document that the -cross images are quite large, and explain why they are so large.

[tianon]

Go 1.5 officially deprecated the -cross variants given that cross-compilation is now trivial, so this no longer applies! 🤘