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-synapse/templates/ext/appservice-discord/systemd/matrix-appservice-discord.service.j2

@@ -10,6 +10,7 @@ After=matrix-synapse.service
 Type=simple
 ExecStartPre=-/usr/bin/docker kill matrix-appservice-discord
 ExecStartPre=-/usr/bin/docker rm matrix-appservice-discord
+
 ExecStart=/usr/bin/docker run --rm --name matrix-appservice-discord \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
@@ -20,12 +21,12 @@ ExecStart=/usr/bin/docker run --rm --name matrix-appservice-discord \
 			{% endif %}
 			-v {{ matrix_appservice_discord_base_path }}:/data \
 			{{ matrix_appservice_discord_docker_image }}
-			
-			
+
 ExecStop=-/usr/bin/docker kill matrix-appservice-discord
 ExecStop=-/usr/bin/docker rm matrix-appservice-discord
 Restart=always
 RestartSec=30
+SyslogIdentifier=matrix-appservice-discord
 
  [Install]
 WantedBy=multi-user.target