Tar-backed transports: oci-archive: (an OCI image layout as a tar) and docker-archive:
(a docker save tarball).
Both are backed by an OCI layout in a temporary work directory: blob and manifest I/O delegate
to Stevedore.Transport.OCILayout, and finalize/1 (called once at the end of a copy) emits
the tar — verbatim for :oci, converted to the docker save layout for :docker. Reading
unpacks the tar into the work dir first, converting from docker save when needed.
Spec: OCI image-layout
and the Docker image spec v1.2 manifest.json.