Ensure networking is up before starting docker

This resolves a problem that I have been having where docker starts before networking is up. See issue #5944 for more details.

Docker-DCO-1.1-Signed-off-by: Jeffrey Bolle <jeffreybolle@gmail.com> (github: jeffreybolle)

contrib/init/upstart/docker.conf

@@ -1,6 +1,6 @@
 description "Docker daemon"
 
-start on local-filesystems
+start on (local-filesystems and net-device-up IFACE!=lo)
 stop on runlevel [!2345]
 limit nofile 524288 1048576
 limit nproc 524288 1048576