Bump to the latest version of skopeo
vendor.conf: add ostree-go
it is used by containers/image for pulling images to the OSTree storage.
fail early when image os does not match host os
Improve documentation on what to do with containers/image failures in test-skopeo
We now have the docker-archive: transport
Integration tests with built registries also exist
Support /etc/docker/certs.d
update image-spec to v1.0.0-rc6
