We hardcode oci-archive for now and use the first file in the pipeline
tree as the archive. Long term we may want to get this info from the
metadata of the oci-archive stage instead.
This adds a stage called org.osbuild.skopeo that installs docker and
oci archive files into the container storage of the tree being
constructed.
The source can either be a file from another pipeline, for example one
created with the existing org.osbuild.oci-archive stage, or it can
be using the new org.osbuild.skopeo source and org.osbuild.containers
input, which will download an image from a registry and install that.
There is an optional option in the install stage that lets you
configure a custom storage location, which allows the use of the
additionalimagestores option in the container storage.conf
to use a read-only image stores (instead of /var/lib/container).
Note: skopeo fails to start if /etc/containers/policy.json is
not available, so we bind mount it from the build tree to the
buildroot if available.
