Fix writing signatures for updated multi-arch image components
Fix writing signatures for updated multi-arch image components

If a per-arch image from a multi-arch image has had its manifest
updated, use the updated manifest digest for PutSignatures.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image