Be explicit about the UID/GID we start Synapse with

We do match the defaults anyway (by default that is),
but people can customize `matrix_user_uid` and `matrix_user_uid`
and it wouldn't be correct then.

In any case, it's better to be explicit about such an important thing.

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

@@ -23,6 +23,8 @@ ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			--network={{ matrix_docker_network }} \
 			-e SYNAPSE_CONFIG_PATH=/data/homeserver.yaml \
 			-e SYNAPSE_CACHE_FACTOR={{ matrix_synapse_cache_factor }} \
+			-e UID={{ matrix_user_uid }} \
+			-e GID={{ matrix_user_gid }} \
 			{% if matrix_synapse_federation_enabled %}
 			-p 8448:8448 \
 			{% endif %}