Add buildpack support

[jamesward]

If there is no Dockerfile, fallback to trying buildpacks. See #3 for an add-on / related discussion.

[poy]

Were you thinking this would be done via Cloud Build or just by invoking pack locally?

[jamesward]

Pack locally. It should be easy to vendor in the binary to the container.

[poy]

Good deal. We will need to wait for buildpacks/pack#209 to be resolved in a release if we want to use an official released version. Otherwise we'll also have to include docker in the image.

[jamesward]

The docker daemon is accessible on Cloud Shell. Right now the button runs docker and it works great. I’ve also tried pack in Cloud Shell and it works great.

[poy]

Ah perfect!

[ahmetb]

Can we do the fallback ONLY if Procfile exists?

I don’t wanna error out saying “pack failed” because pack is 5% of the use case or less. I mostly want to say “build failed; no Dockerfile or Procfile” instead.

[poy]

This implies you have to have a procfile for it to use buildpacks. This isn't really necessary as the buildpacks can create procfile for you.