The new OCI-compatible archive export relies on the Descriptors returned
by the layer (`distribution.Describable` interface implementation).
The issue with that is that the `roLayer` and the `referencedCacheLayer`
types don't implement this interface. Implementing that interface for
them based on their `descriptor` doesn't work though, because that
descriptor is empty.
To workaround this issue, just create a new descriptor if the one
provided by the layer is empty.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
2c45df2ab4