Add an option to copy foreign layers

[mduft]

I would like to skopeo copy a windows image to a linux host to create (via pure filesystem manipulation with umoci) new layers. Windows is dead slow with a lot of files, so I'd rather use linux :) I don't need to be able to run any commands, thus I don't need support for Dockerfile, RUN, etc. I just want to add files, and commit a new layer.

Trying out skopeo to do this, I unfortunately get:

skopeo --override-os windows --override-arch amd64 copy docker://microsoft/windowsservercore:ltsc2016 oci:windows-test:latest
Getting image source signatures
Skipping foreign layer "sha256:3889bb8d808bbae6fa5a33e07093e65c31371bcf9e4c38c21be6b9af52ad1548" copy to oci
Skipping foreign layer "sha256:6631c2d2a60cd7ee5b334c2725b03e4d4976abb9a19c8e8dc9b806b3752745a6" copy to oci
Copying config sha256:9657141ae0624406d5e84631b2cab0eee14d2810a7ca29250d32d1babb68101b
 493 B / 493 B [============================================================] 0s
Writing manifest to image destination
Storing signatures

Any advice? Should this work?