String getLayerFilename(DescriptorDigest layerDiffId) {
    return layerDiffId.getHash();
  }
Gets the filename for the layer file. The filename is in the form {@code <layer diff
ID>.layer}.

@param layerDiffId the layer's diff ID
@return the layer filename
jib-core/src/main/java/com/google/cloud/tools/jib/cache/CacheStorageFiles.java