Prevent 404s when Riot tries to retrieve domain-specific config.json

Riot unconditionally asks for a config.${document.domain}.json, c.f.
https://github.com/vector-im/riot-web/blame/develop/src/vector/getconfig.ts#L24

roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2

@@ -24,6 +24,7 @@ ExecStart=/usr/bin/docker run --rm --name matrix-riot-web \
 			-v {{ matrix_riot_web_data_path }}/nginx.conf:/etc/nginx/nginx.conf:ro \
 			-v /dev/null:/etc/nginx/conf.d/default.conf:ro \
 			-v {{ matrix_riot_web_data_path }}/config.json:/app/config.json:ro \
+			-v {{ matrix_riot_web_data_path }}/config.json:/app/config.{{ matrix_server_fqn_riot }}.json:ro \
 			{% if matrix_riot_web_embedded_pages_home_path is not none %}
 			-v {{ matrix_riot_web_data_path }}/home.html:/app/home.html:ro \
 			{% endif %}