How to use agouti on google app engine

[cuzkop]

hello from japan

I want to use agouti on gae like this

	capabilities := agouti.NewCapabilities()
	capabilities["phantomjs.page.settings.userAgent"] = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_3) AppleWebKit/###.## (KHTML, like Gecko) Chrome/##.#.####.### Safari/###.##"
	capabilitiesOption := agouti.Desired(capabilities)

	driver := agouti.PhantomJS(capabilitiesOption)
	if err := driver.Start(); err != nil {
		log.Errorf(c, "Failed to retrieve next log: %v", err)
	}

But it will become error like this

failed to start service: failed to run command: exec: "phantomjs": executable file not found in $PATH

what happened?
and how to use agouti?

I'm sorry for my cheap English