[RFC #0105] Rebaser changes for run image extension (Dockerfiles phase 3)

[natalieparellano]

Spec PR: https://github.com/buildpacks/spec/pull/347/files

For newer platform API (0.12 and above), the rebaser should require -force if the image has the label io.buildpacks.rebasable set to false.

Additionally:

- **If** `<run-image>` is not provided by the platform, the value will be [resolved](#run-image-resolution) from the contents of the `runImage` key in the `io.buildpacks.lifecycle.metdata` label on `<image>`

Thus instead of stack.runImage.image we use runImage.image on the label (note the schema change in the spec PR).