Run Element Web in tightened/read-only mode without a custom nginx config

Newer Element Web versions allow for the nginx port to be
overriden, etc., and provide instructions for running in read-only mode.

This makes our custom `nginx.conf` patches unnecessary.

Passing the correct `ELEMENT_WEB_PORT` environment variable
also helps with future changes.

Another benefit of this (besides keeping closer to upstream
recommendations and the improved simplicity) is that:

- the container can run its entrypoint env-substitutions code now,
  without reporting errors

- IPv6 for nginx works, so `matrix-client-element:8080` is accessible
  via IPv6 on the container network now
  (this affects only for Traefik's communicaton with Element Web
  internally; public connectivity was handled by Traefik and IPv6 was
  available there even before)

Ref:

- 2052080d7d/docs/install.md (docker)
- https://github.com/element-hq/element-web/pull/28849
- https://github.com/element-hq/element-web/pull/28840

roles/custom/matrix-client-element/templates/systemd/matrix-client-element.service.j2

@@ -22,11 +22,15 @@ ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} create \
 			--read-only \
 			--network={{ matrix_client_element_container_network }} \
 			{% if matrix_client_element_container_http_host_bind_port %}
-			-p {{ matrix_client_element_container_http_host_bind_port }}:8080 \
+			-p {{ matrix_client_element_container_http_host_bind_port }}:{{ matrix_client_element_container_port }} \
 			{% endif %}
 			--label-file={{ matrix_client_element_data_path }}/labels \
+			--env-file={{ matrix_client_element_data_path }}/env \
 			--tmpfs=/tmp:rw,noexec,nosuid,size=10m \
-			--mount type=bind,src={{ matrix_client_element_data_path }}/nginx.conf,dst=/etc/nginx/nginx.conf,ro \
+			--tmpfs=/var/cache/nginx:rw,mode=777 \
+			--tmpfs=/var/run:rw,mode=777 \
+			--tmpfs=/tmp/element-web-config:rw,mode=777 \
+			--tmpfs=/etc/nginx/conf.d:rw,mode=777 \
 			--mount type=bind,src={{ matrix_client_element_data_path }}/config.json,dst=/app/config.json,ro \
 			--mount type=bind,src={{ matrix_client_element_data_path }}/config.json,dst=/app/config.{{ matrix_server_fqn_element }}.json,ro \
 			{% if matrix_client_element_location_sharing_enabled %}