fix: hardcode OO domain in Traefik label; Coolify does not expand vars in labels

- nextcloud/docker-compose.yml: X-Forwarded-Host=onlyoffice.sherlockhomeless.net
  (was ${OO_DOMAIN} which Coolify leaves unexpanded → OnlyOffice loads assets
  from https://${oo_domain}/ and editor breaks entirely)
- nextcloud/stack.env: update placeholder domains to real ones
- COOLIFY-TEMPLATE.md: add Gotcha 6 about label variable expansion

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

COOLIFY-TEMPLATE.md

@@ -235,3 +235,21 @@ ls /opt/<stack>/     # ej: /opt/adguard/, /opt/authentik/, /opt/gitea/, etc.
 
 Si existe el directorio con datos → usar bind mount a esa ruta.
 Si no existe → crear el directorio antes de desplegar, o usar named volume.
+
+### Gotcha 6 — Variables en labels de Traefik NO se expanden en Coolify
+
+Coolify expande `${VAR}` en la sección `environment:` pero NO en `labels:`.
+
+```yaml
+# ❌ MAL: quedará como literal ${OO_DOMAIN} en el label del contenedor
+labels:
+  - traefik.http.middlewares.foo.headers.customRequestHeaders.X-Forwarded-Host=${OO_DOMAIN}
+
+# ✅ BIEN: hardcodear el valor real
+labels:
+  - traefik.http.middlewares.foo.headers.customRequestHeaders.X-Forwarded-Host=onlyoffice.sherlockhomeless.net
+```
+
+Esto afecta especialmente a headers `X-Forwarded-Host` de OnlyOffice — si queda
+como literal, el JS de OnlyOffice intenta cargar assets de `https://${oo_domain}/...`
+y el editor de documentos falla completamente.