Support for image chaining / add base in apko_image

[iDiogenes]

Thank you for creating this Bazel rule; it was exactly what I was looking for. However, I have one question: Is there, or will there be, support for using a previously built image? Or does apko_image always necessitate bootstrapping the image from scratch? It would be beneficial if we could chain images together, adding packages incrementally to meet the varying requirements of our services.

In oci_image you can set a base which can be the output from another oci_image. It would be fantastic if apko_image supported something like this as well.

I suspect this request also belongs in https://github.com/chainguard-dev/apko/issues as rules_apko just wraps apko, but I wanted to make sure I was not missing something that can be done with bazel.

Thanks again!