Add network option for image building

This commit adds configuration to the Maven and Gradle plugins to
allow specifying the network mode to be provided to the image
building goal and task.

See gh-27486

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/BootBuildImageTests.java

@@ -42,6 +42,7 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
  * @author Andy Wilkinson
  * @author Scott Frederick
  * @author Andrey Shlykov
+ * @author Jeroen Meijer
  */
 class BootBuildImageTests {
 
@@ -278,4 +279,10 @@ class BootBuildImageTests {
 				.containsExactly(Binding.of("host-src:container-dest:ro"), Binding.of("volume-name:container-dest:rw"));
 	}
 
+	@Test
+	void whenNetworkIsConfiguredThenRequestHasNetwork() {
+		this.buildImage.setNetwork("test");
+		assertThat(this.buildImage.createRequest().getNetwork()).isEqualTo("test");
+	}
+
 }