emit a warning if old-style config.php is detected

src/app/startup.sh

@@ -71,6 +71,9 @@ if [ ! -s $DST_DIR/config.php ]; then
 	cat >> $DST_DIR/config.php << EOF
 		define('NGINX_XACCEL_PREFIX', '/tt-rss');
 EOF
+else
+	egrep 'SELF_URL_PATH.*getenv' $DST_DIR/config.php || \
+		echo -e "\nWARNING: you're using old-style config.php, overrides via .env will not work.\n" >/dev/stderr
 fi
 
 # this was previously generated