Do not build using sudo on Mac OS X

Building using sudo fails on Mac OS X, but is necessary on Linux.

See https://groups.google.com/d/topic/docker-dev/1EW_pfA1t9o/discussion for discussion.

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)
Merge pull request #6304 from glyn/mac-build-no-sudo

Do not build using sudo on Mac OS X