Provenance for images built with other tooling

[jeremyrickard]

There is a brief mention of non Dockerfile driven workflows with the buildType attribute, but there aren't any examples or narrative that explain how we expect this to work for non Dockerfile based things (i.e. Bazel, or cloud native build packs, or ko or other things that simply create a tgz of a layer and either append to an existing thing or build an entirely new manifest around it).

Is the initial scope for this going to be strictly limited to things in the realm of Buildkit / Dockerfile? Are there any important use cases within Azure services that would get missed by that assumption?