Update Dockerfile example.

[sidharthv96]

I just spent more than an hour trying to debug why I wasn't able to deploy my app. I copied the example and just removed the my. Didn't even notice that the F was capitalized.

Finally had to clone the two repos, setup the dev environment, debug ImageMaker to find out what the error was.

I hope this will save someone else from having to go through the same ordeal :)

[githubsaturn]

Thanks!