static DescriptorDigest generateSelector(ImmutableList<FileEntry> layerEntries)
      throws IOException {
    return Digests.computeJsonDigest(toSortedJsonTemplates(layerEntries));
  }
Generates a selector for the list of {@link FileEntry}s. The selector is unique to each unique
set of layer entries, regardless of order. TODO: Should we care about order?

@param layerEntries the layer entries
@return the selector
@throws IOException if an I/O exception occurs
jib-core/src/main/java/com/google/cloud/tools/jib/cache/LayerEntriesSelector.java