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