Add slightly better GOPATH detection/handling

This also adds a new "AUTO_GOPATH" environment variable that will create an appropriate GOPATH as part of the build process.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4031 from tianon/gopath

Add slightly better GOPATH detection/handling