Support Dockerfiles (phases 1 and 2)

[natalieparellano]

CNB RFC 105 introduces the concept of image extensions to the CNB spec. Image extensions output Dockerfiles that can be used to extend build or run-time base images prior to buildpack execution.

Because of the complexity involved, this feature will be implemented using a phased approach (more information can be found here). Lifecycle 0.15.0 will have support for phases 1 and 2.

We should discuss what it would look like to support this feature in kpack. From what I understand, the ability to switch the run image from the one defined on the builder resource will be the most complicated aspect, but there could be other things unforeseen.