`--pull`: Add the host container storage as a read-only additional image store

_Originally posted by @cgwalters in https://github.com/containers/bootc/pull/860#discussion_r1824410688_

>One thing that might be a nice followup is that we add the host container storage as a read-only additional image store; I think this would get us "pull if not present" semantics.

Start by tracing the `--pull` command path and how image stores are selected. Determine how the host container storage can be added as a read-only additional image store, then verify that pulling uses the host copy when the image is already present.

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