contrib: systemd: make socket-activation Restart=always

Do as was done to f09a78cd219b24d4308034c8dd13410cfe5fbec7 in the
socket-activation example.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)

contrib/init/systemd/socket-activation/docker.service

@@ -5,6 +5,7 @@ After=network.target
 
 [Service]
 ExecStart=/usr/bin/docker -d -H fd://
+Restart=on-failure
 
 [Install]
 WantedBy=multi-user.target