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:

- https://github.com/element-hq/element-web/blob/2052080d7d8a213064910cac491ec5cf9057610e/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/env.j2

@@ -0,0 +1 @@
+ELEMENT_WEB_PORT={{ matrix_client_element_container_port }}