Fix the systemd socket activation socket permissions

Fixes #6836

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Upstream-commit: 3589f5b9c2d9e5cb6d057713aa8d591295c5f523
Component: engine

components/engine/contrib/init/systemd/socket-activation/docker.socket

@@ -3,6 +3,9 @@ Description=Docker Socket for the API
 
 [Socket]
 ListenStream=/var/run/docker.sock
+SocketMode=0660
+SocketUser=root
+SocketGroup=docker
 
 [Install]
 WantedBy=sockets.target