Zulip Server 12.1-0

• Update to Zulip Server 12.1.
• Bake the /.dockerenv marker into the image, so Zulip's
  RUNNING_IN_DOCKER detection works under container runtimes that
  don't create it themselves (containerd, CRI-O). This makes
  configuration error messages point at the SETTING_* environment
  variables rather than at /etc/zulip/settings.py, which is
  especially relevant for Kubernetes deployments.
• Reject environment variables from the legacy zulip/docker-zulip
  image at startup. Names that were renamed or removed in the move to
  ghcr.io/zulip/zulip-server (e.g. DB_HOST, DISABLE_HTTPS,
  NGINX_WORKERS) were previously silently ignored; the entrypoint
  now exits with a diagnostic naming the new spelling (or flagging it
  as removed) and a pointer to the legacy-upgrade docs.
• New documentation:
  • How-to guide for placing your own Ingress controller, Gateway
    API implementation, or service mesh in front of Zulip, and which
    proxy-adjacent settings remain application configuration.
  • Note on when the bundled RabbitMQ subchart may fail to start on
    hardened Kubernetes clusters, pointing to external RabbitMQ as
    the robust alternative.
  • Added missing cross-links.