public Optional<String> getGroup() {
    return Optional.ofNullable(group);
  }
Constructor for use by jackson to populate this object.

@param filePermissions octal string for file permissions
@param directoryPermissions octal string for directory permissions
@param user name or number for ownership user
@param group name or number for ownership group
@param timestamp in milliseconds since epoch or ISO 8601 datetime
jib-cli/src/main/java/com/google/cloud/tools/jib/cli/buildfile/FilePropertiesSpec.java