idhub.entrypoint: transition to using default port

docker/idhub.entrypoint.sh

@@ -99,6 +99,7 @@ config_oidc4vp() {
 }
 
 runserver() {
+        PORT="${PORT:-8000}"
         if [ ! "${DEBUG:-}" = "true" ]; then
                 ./manage.py collectstatic
                 if [ "${EXPERIMENTAL:-}" = "true" ]; then