Systemd .service should install .socket

And remove the WantedBy directive.

See: https://bugs.archlinux.org/task/41338

Docker-DCO-1.1-Signed-off-by: Sébastien Luttringer <seblu@seblu.net> (github: seblu)

contrib/init/systemd/docker.service

@@ -11,4 +11,4 @@ LimitNOFILE=1048576
 LimitNPROC=1048576
 
 [Install]
-WantedBy=multi-user.target
+Also=docker.socket