Add the possibility to pass extra flags to the docker container

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2

@@ -46,6 +46,9 @@ ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			{% for volume in matrix_synapse_container_additional_volumes %}
 			-v {{ volume.src }}:{{ volume.dst }}:{{ volume.options }} \
 			{% endfor %}
+			{% for arg in matrix_synapse_container_extra_arguments %}
+			{{ arg }} \
+			{% endfor %}
 			{{ matrix_synapse_docker_image }} \
 			-m synapse.app.homeserver -c /data/homeserver.yaml