I would love to use `hack/make.sh` verbatim to compile Docker in the Gentoo ebuild, but this check to ensure we're inside docker precludes that.  I'm not opposed to copying the `VERSION`, `GITCOMMIT`, and `LDFLAGS` lines manually and then just using `hack/make/binary`, if that's the route I should go, but otherwise really enjoying this change.

I think it would also be helpful if I could add extra build flags to "go build" via some kind of environment variable.  I've found that doing the "go test" hack to get the standard libraries compiled without CGO for our static binary writes the newly compiled versions directly in /usr/share, where just supplying "-a" to the go build command accomplishes the same task without touching /usr/share.