public static ImageReference scratch() {
    return new ImageReference("", SCRATCH, null, null);
  }
Constructs an {@link ImageReference} with an empty registry and tag component, and repository
set to "scratch".

@return an {@link ImageReference} with an empty registry and tag component, and repository set
    to "scratch"
jib-core/src/main/java/com/google/cloud/tools/jib/api/ImageReference.java