@Test
  public void testRunPluginExtensions_customProperties() throws JibPluginExtensionException {
    FooExtension extension =
        new FooExtension(
            (buildPlan, properties, extraConfig, gradleData, logger) ->
                buildPlan.toBuilder().setUser(properties.get("user")).build());
    loadedExtensions = Arrays.asList(extension);

    JibContainerBuilder extendedBuilder =
        gradleProjectProperties.runPluginExtensions(
            Arrays.asList(new FooExtensionConfig(ImmutableMap.of("user", "65432"))),
            containerBuilder);
    Assert.assertEquals("65432", extendedBuilder.toContainerBuildPlan().getUser());
  }
Plugin extension test for {@link GradleProjectProperties}.
jib-gradle-plugin/src/test/java/com/google/cloud/tools/jib/gradle/GradleProjectPropertiesExtensionTest.java