public CachedLayer build() {
      return new CachedLayer(
          Preconditions.checkNotNull(layerDigest, "layerDigest required"),
          Preconditions.checkNotNull(layerDiffId, "layerDiffId required"),
          layerSize,
          Preconditions.checkNotNull(layerBlob, "layerBlob required"));
    }
Creates a CachedLayer instance.

@return a new cached layer
jib-core/src/main/java/com/google/cloud/tools/jib/cache/CachedLayer.java