chore(compose): delegate port routing to Coolify reverse proxy

Replace host port binding with expose so Coolify's Traefik handles
external routing. Remove PORT env var as it is no longer needed.

.env.example

@@ -1,2 +1 @@
 DOCLING_SERVE_ENABLE_UI=true
-PORT=5001

docker-compose.yml

@@ -1,8 +1,8 @@
 services:
   docling:
     image: quay.io/docling-project/docling-serve:latest
-    ports:
+    expose:
-      - "${PORT:-5001}:5001"
+      - "5001"
     environment:
       - DOCLING_SERVE_ENABLE_UI=${DOCLING_SERVE_ENABLE_UI:-true}
     healthcheck: