Polish "Add network option for image building"

See gh-27486

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-buildsImageWithNetworkModeNone.gradle

@@ -0,0 +1,16 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+if (project.hasProperty('applyWarPlugin')) {
+	apply plugin: 'war'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	builder = "projects.registry.vmware.com/springboot/spring-boot-cnb-builder:0.0.1"
+	network = "none"
+}