systemd unitfile requires docker.socket

modified:   contrib/init/systemd/docker.service

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org> (github: lsm5)

contrib/init/systemd/docker.service

@@ -2,6 +2,7 @@
 Description=Docker Application Container Engine
 Documentation=http://docs.docker.com
 After=network.target
+Requires=docker.socket
 
 [Service]
 ExecStart=/usr/bin/docker -d -H fd://