Support copying images for specific platforms/architectures about image

[mtrmac]

Thanks for reaching out.

Right now, there is copy.CopySpecificImages, but that requires the caller to manually determine which images to copy.

And overall, the ImageListSelection affects which per-platform instances are copied, but the whole manifest list is used, possibly referring to missing per-platform instances. Some registries reject that.

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It would make sense to provide both of those features

• to let the caller provide some kind of platform filter, without having to individually worry about instances manually. We need to worry about all three fields determining the platform: OS/architecture/variant, and possibly more fields that will be added.
• to allow modifying the manifest to refer only to the copied per-platform instances, instead of writing a sparse manifest. That would require building rather more of the manifest editing infrastructure (or moving Podman’s implementation).

[mtrmac]

Modifying the manifest is happening in #1707 .