@Input
  @Nullable
  @Optional
  public String getMainClass() {
    String mainClassProperty = System.getProperty(PropertyNames.CONTAINER_MAIN_CLASS);
    if (mainClassProperty != null) {
      return mainClassProperty;
    }
    return mainClass.getOrNull();
  }
A bean that configures properties of the container run from the image. This is configurable with
Groovy closures and can be validated when used as a task input.
jib-gradle-plugin/src/main/java/com/google/cloud/tools/jib/gradle/ContainerParameters.java