@VisibleForTesting
  @Nullable
  static Map<String, Map<String, String>> volumesSetToMap(@Nullable Set<AbsoluteUnixPath> volumes) {
    return setToMap(volumes, AbsoluteUnixPath::toString);
  }
Converts a set of {@link AbsoluteUnixPath}s to the corresponding container config format for
volumes (e.g. {@code AbsoluteUnixPath().get("/var/log/my-app-logs")} -> {@code
{"/var/log/my-app-logs":{}}}).

@param volumes the set of {@link AbsoluteUnixPath}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