Support cloudbuild.yaml

It seems like we want to support cloudbuild.yaml in addition to Dockerfiles, and there's a plan to support buildpacks via app.json (#3).

@jamesward What's the primary reason we want to have cloudbuild.yaml? Does it do something potentially we can’t do via `docker build` with multi-stage Dockerfiles?

I suppose it enables tools where you don't want to use `docker` and still end up with a docker image (`ko`, `pack` etc.), but should we treat this as a priority –especially given the adoption levels of cloudbuild.yaml vs Dockerfiles?