adds support to manifest to have a from step.
the buildtool has a separate "FROM" run, downloading an image to a local oci-dir,
and then passing it as arguments to create-manifest and create-layout
adds support to manifest to have a from step.
the buildtool has a separate "FROM" run, downloading an image to a local oci-dir,
and then passing it as arguments to create-manifest and create-layout