Fix OnlyOffice→Seafile connectivity (hairpin NAT)

Add extra_hosts: docs.alogins.net:host-gateway so OnlyOffice container
can reach Seafile's callback URL without going through the public IP.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

seafile/onlyoffice.yml

@@ -10,6 +10,8 @@ services:
       - "${ONLYOFFICE_VOLUME:-/opt/onlyoffice}:/var/lib/onlyoffice"
     ports:
       - "127.0.0.1:6233:80"
+    extra_hosts:
+      - "docs.alogins.net:host-gateway"
     networks:
       - seafile-net