Change default port to 20211 as in docker container

dockerfiles/start.sh

@@ -45,8 +45,11 @@ ln -s $INSTALL_DIR/pialert/front /var/www/html/pialert
 sudo rm /etc/nginx/conf.d/pialert.conf
 # create symbolic link to NGINX configuaration coming with PiAlert
 sudo ln -s "$INSTALL_DIR/pialert/install/pialert.conf" /etc/nginx/conf.d/pialert.conf
-# use user-supplied port
-sudo sed -i 's/listen 80/listen '"$PORT"'/g' /etc/nginx/conf.d/pialert.conf
+
+# Use user-supplied port if set
+if [ -n "${PORT}" ]; then  
+  sudo sed -i 's/listen 20211/listen '"$PORT"'/g' /etc/nginx/conf.d/pialert.conf
+fi
 
 # Change web interface address if set
 if [ -n "${LISTEN_ADDR}" ]; then