Use explicit GOPATH in Makefile (don't inherit a GOPATH, which helps ensure consistent builds, especially since the GOPATH is a colon-separated list of directories, and the variable gets used directly in other places for creating paths)
Ensure proper failure of Makefile's "go get" when directory fails to exist
Add symlink code back a little safer to save us from downloading the docker repo again when we already have a local copy we're trying to build