Set generic gid to be consistent with uid on OS-X.
Set generic gid to be consistent with uid on OS-X.

See https://github.com/larskanis/rake-compiler-dock/pull/2.

larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock