Use SyslogIdentifier to improve logging

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2

@@ -44,6 +44,7 @@ ExecStop=-/usr/bin/docker rm matrix-nginx-proxy
 ExecReload=/usr/bin/docker exec matrix-nginx-proxy /usr/sbin/nginx -s reload
 Restart=always
 RestartSec=30
+SyslogIdentifier=matrix-nginx-proxy
 
 [Install]
 WantedBy=multi-user.target