Dockerfile pulls directly from canvas-lms's git master about canvas-docker

[lbjay]

Thanks for the patch. I haven't had the capacity to deal with the broken build lately, but yeah, it's not working as intended. The idea was to trigger the build based on activity in the canvas-lms repo, but I didn't manage to get the right combination of github activity feed + build trigger params. And AFAIK you still can't pass build args to docker hub automated builds anyway.

In the interests of getting a working build I try to get the patch incorporated.

[lbjay]

Hi David,
Thanks again for the suggested changes. I've incorporated some of them (plus a couple more) in f78d854. I've also switched to using AWS CodeBuild for building the image which gives a little more control over which upstream revision tag to use as well as how to tag the resulting image. So now there is a working "release-2017-06-03.14" image as well as "latest".