@VisibleForTesting
  @Nullable
  static Map<String, Map<String, String>> portSetToMap(@Nullable Set<Port> exposedPorts) {
    return setToMap(exposedPorts, port -> port.getPort() + "/" + port.getProtocol());
  }
Converts a set of {@link Port}s to the corresponding container config format for exposed ports
(e.g. {@code Port(1000, Protocol.TCP)} -> {@code {"1000/tcp":{}}}).

@param exposedPorts the set of {@link Port}s to translate, or {@code null}
@return a sorted map with the string representation of the ports as keys and empty maps as
    values, or {@code null} if {@code exposedPorts} is {@code null}
jib-core/src/main/java/com/google/cloud/tools/jib/image/json/ImageToJsonTranslator.java