Add the possibility to pass extra flags to the docker container

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

@@ -18,6 +18,9 @@ ExecStart=/usr/bin/docker run --rm --name matrix-postgres \
 			--env-file={{ matrix_postgres_base_path }}/env-postgres-server \
 			-v {{ matrix_postgres_data_path }}:/var/lib/postgresql/data:rw \
 			-v /etc/passwd:/etc/passwd:ro \
+			{% for arg in matrix_postgres_container_extra_arguments %}
+			{{ arg }} \
+			{% endfor %}
 			{{ matrix_postgres_docker_image_to_use }}
 ExecStop=-/usr/bin/docker stop matrix-postgres
 ExecStop=-/usr/bin/docker rm matrix-postgres