@Override
  public Blob getBlob() throws LayerPropertyNotFoundException {
    throw new LayerPropertyNotFoundException("Blob not available for digest-only layer");
  }
Instantiate with a {@link DescriptorDigest}.

@param digest the digest to instantiate the {@link DigestOnlyLayer} from
jib-core/src/main/java/com/google/cloud/tools/jib/image/DigestOnlyLayer.java