@Nullable
  String getMainClass() {
    String property = getProperty(PropertyNames.CONTAINER_MAIN_CLASS);
    if (property != null) {
      return property;
    }
    return container.mainClass;
  }
Gets the name of the main class.

@return the configured main class name
jib-maven-plugin/src/main/java/com/google/cloud/tools/jib/maven/JibPluginConfiguration.java