io.buildpacks.builder.metadata is incorrect for builders that declare [[run.images]] in builder.toml

[natalieparellano]

Summary

This is our fault for not having a builder spec. But when we added [[run.images]] to builder.toml (which results in the run.toml file getting created within the builder) we didn't spec how that information would be declared in the io.buildpacks.builder.metadata label.

For reference, stack information is declared in that label with schema:

  "stack": {
    "runImage": {
      "image": "some-run-image",
      "mirrors": []
    }
  }

We probably want to declare run images information in that label with schema:

  "run": {
    "images": [
      {
        "image": "some-run-image",
        "mirrors": []
      }
    ]
  }

But again, we don't have a builder spec. Today, due to some struct sharing, that information is encoded as:

  "images": [
    {
      "image": "some-run-image",
      "mirrors": []
    }
  ]

This is confusing. We should also consider whether we want to encode any information about the build-time base (in absence of a stack ID this information is lost).