Without this `ctr run` can fail with:
ctr: parent snapshot sha256:70798fd80095f40b41baa5d107fb61532bfe494d96313fea01e8fcbf4e8743ee does not exist: not found
My image was produced by buildkit, which doesn't unpack (I think this makes
sense since buildkit doesn't know if I am going to run the image or export/push
it etc).
Signed-off-by: Ian Campbell <ian.campbell@docker.com>