Fix dockerImageDestination.SupportsSignatures
Fix dockerImageDestination.SupportsSignatures

Apparently we have never been able to push signatures to sigstore,
and nobody noticed?!

FIXME: This needs a test

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

masters-of-cats/image,containers/image,containers/image