Path getLayerDirectory(DescriptorDigest layerDigest) {
    return getLayersDirectory().resolve(layerDigest.getHash());
  }
Gets the directory for the layer with digest {@code layerDigest}.

@param layerDigest the digest of the layer
@return the directory for that {@code layerDigest}
jib-core/src/main/java/com/google/cloud/tools/jib/cache/CacheStorageFiles.java