Caddy Server and Nginx. Get output of reverse proxy if static file not found

caddyfile

http://localhost:8080 {
	root * ./public

	@notStatic {
		not {
			file {
				try_files {path} {path}/
			}
		}
	}

	# or short check.
	#@notStatic not file

	# serve the app if there's not a static file.
	reverse_proxy @notStatic http://localhost:9000

	# Serve the static file otherwise.
	file_server
}

nginx.conf

location / {
  try_files $uri $uri/ @backend;
}

location @backend {
  proxy_pass http://localhost:3000;
}