Re #2137 Note about vagrant dns in the test documentation

docs/sources/contributing/devenvironment.rst

@@ -78,6 +78,8 @@ To execute the test cases, run this command:
 	sudo docker run -lxc-conf=lxc.aa_profile=unconfined -privileged -v `pwd`:/go/src/github.com/dotcloud/docker docker hack/make.sh test
 
 
+Note: if you're running the tests in vagrant, you need to specify a dns entry in the command: `-dns 8.8.8.8`
+
 If the test are successful then the tail of the output should look something like this
 
 .. code-block:: bash