Enable configuration options that can selectively disable validation
that dependencies exist within the registry before the image index
is uploaded.
This enables sparse indexes, where a registry holds a manifest index that
could be signed (so the digest must not change) but does not hold every
referenced image in the index. The use case for this is when a registry
mirror does not need to mirror all platforms, but does need to maintain
the digests of all manifests either because they are signed or because
they are pulled by digest.
The registry administrator can also select specific image architectures
that must exist in the registry, enabling a registry operator to select
only the platforms they care about and ensure all image indexes uploaded
to the registry are valid for those platforms.
Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>
c40c4b289a