Convert to `oci-import` builder?

Now that DOI has a means of importing OCI images as-is, our non-Windows images are no longer getting much value out of being built from a Dockerfile, and we could convert that metadata directly into JSON instead.

See:

• https://github.com/docker-library/bashbrew/pull/51
• https://github.com/docker-library/bashbrew/pull/61
• https://github.com/docker-library/official-images/pull/13769
• https://github.com/docker-library/meta-scripts/pull/20
• https://github.com/docker-library/busybox/pull/188

It's probably worthwhile to consider this together with #122 (because it'll be some amount of repository churn), and might actually be a good use case for the File: support in the oci-import builder that so far hasn't had any actual load-bearing uses yet.

It's probably also worth considering doing uncompressed tarballs, since our binaries are already minimal, but it's also possible we get reasonable compression, so it's something that could be considered (we simply shouldn't rule out doing uncompressed tarballs without actually evalutating it).

Review the linked bashbrew, official-images, meta-scripts, and busybox pull requests, then compare #122 with the current Dockerfile-based metadata. Evaluate oci-import's File: support and compressed versus uncompressed tarballs; done means an agreed conversion path for the non-Windows images.