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-maven-plugin/src/test/java/org/springframework/boot/maven/ImageTests.java

@@ -41,6 +41,7 @@ import static org.assertj.core.api.Assertions.entry;
  *
  * @author Phillip Webb
  * @author Scott Frederick
+ * @author Jeroen Meijer
  */
 class ImageTests {
 
@@ -70,6 +71,7 @@ class ImageTests {
 		assertThat(request.getPullPolicy()).isEqualTo(PullPolicy.ALWAYS);
 		assertThat(request.getBuildpacks()).isEmpty();
 		assertThat(request.getBindings()).isEmpty();
+		assertThat(request.getNetwork()).isNull();
 	}
 
 	@Test
@@ -146,6 +148,14 @@ class ImageTests {
 				Binding.of("volume-name:container-dest:rw"));
 	}
 
+	@Test
+	void getBuildRequestWhenNetworkUsesNetwork() {
+		Image image = new Image();
+		image.network = "test";
+		BuildRequest request = image.getBuildRequest(createArtifact(), mockApplicationContent());
+		assertThat(request.getNetwork()).isEqualTo("test");
+	}
+
 	private Artifact createArtifact() {
 		return new DefaultArtifact("com.example", "my-app", VersionRange.createFromVersion("0.0.1-SNAPSHOT"), "compile",
 				"jar", null, new DefaultArtifactHandler());