Fix Nginx template environment variable syntax for Docker deployment

nginx-template.conf

@@ -11,7 +11,7 @@ server {
 
     # Proxy API requests to the backend server
     location /api/ {
-        proxy_pass http://$BACKEND_HOST:$BACKEND_PORT/;
+        proxy_pass http://${BACKEND_HOST}:${BACKEND_PORT}/;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';