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)

docs/sources/contributing/devenvironment.md

@@ -50,6 +50,13 @@ This command will take some time to complete when you first execute it.
 If the build is successful, congratulations! You have produced a clean
 build of docker, neatly encapsulated in a standard build environment.
 
+> **Note**:
+> On Mac OS X, make targets such as `build`, `binary`, and `test`
+> must **not** be built under root. So, for example, instead of the above
+> command, issue:
+> 
+>     $ make build
+
 ## Build the Docker Binary
 
 To create the Docker binary, run this command: