Change docker socket location to /run/docker.sock

This change resolves the following systemd warning:

```
/usr/lib/systemd/system/docker.socket:5: ListenStream= references a path below legacy directory /var/run/, updating /var/run/docker.sock → /run/docker.sock; please update the unit file accordingly.
```

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

contrib/init/systemd/docker.socket

@@ -3,7 +3,9 @@ Description=Docker Socket for the API
 PartOf=docker.service
 
 [Socket]
-ListenStream=/var/run/docker.sock
+# If /var/run is not implemented as a symlink to /run, you may need to
+# specify ListenStream=/var/run/docker.sock instead.
+ListenStream=/run/docker.sock
 SocketMode=0660
 SocketUser=root
 SocketGroup=docker