Update hack/make.sh with slightly clearer output and a more precise "are we in the Docker container?" check

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3760 from tianon/better-nondocker-warning

Update hack/make.sh with slightly clearer output and a more precise "are we in the Docker container?" check