stabilization tracker: bootc image copy-to-storage

[cgwalters]

Original PR: #620

This issue tracks the stabilization of bootc image, which currently just has one subcommand bootc image copy-to-storage.

Rationale

We want to make it ergonomic and convenient to operate on the booted host image via e.g. podman. Today usage of podman defaults to operating on /var/lib/containers, a distinct mutable store.

Having the ability to conveniently copy the image to the booted storage aids with https://containers.github.io/bootc/booting-local-builds.html

Concerns

• Longer term, we want to unify bootc and podman storage; but perhaps copy-to-storage can become a no-op
• Would it be better to try to instead expose an API to read from the bootc storage, and wrap that in a CLI?