Use default tag when pulling run image

Prior to this commit, a default tag of 'latest' was used when no tag
was included in the builder image name used when building an image in
the Maven and Gradle plugins, but the tag for the run image was left
empty if it was not provided. This resulted in errors when pulling
the run image from an image repository. This commit applies the
same tag defaulting logic to the run image name.

Fixes gh-21532

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/Builder.java

@@ -34,6 +34,7 @@
  * Central API for running buildpack operations.
  *
  * @author Phillip Webb
+ * @author Scott Frederick
  * @since 2.3.0
  */
 public class Builder {
@@ -89,7 +90,7 @@ private Image pullBuilder(BuildRequest request) throws IOException {
 	private ImageReference getRunImageReference(Stack stack) {
 		String name = stack.getRunImage().getImage();
 		Assert.state(StringUtils.hasText(name), "Run image must be specified");
-		return ImageReference.of(name);
+		return ImageReference.of(name).inTaggedForm();
 	}
 
 	private Image pullRunImage(BuildRequest request, ImageReference name) throws IOException {

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/build/BuilderTests.java

@@ -86,6 +86,26 @@ void buildInvokesBuilder() throws Exception {
 		verify(docker.image()).remove(archive.getValue().getTag(), true);
 	}
 
+	@Test
+	void buildInvokesBuilderWithDefaultImageTags() throws Exception {
+		TestPrintStream out = new TestPrintStream();
+		DockerApi docker = mockDockerApi();
+		Image builderImage = loadImage("image-with-no-run-image-tag.json");
+		Image runImage = loadImage("run-image.json");
+		given(docker.image().pull(eq(ImageReference.of("gcr.io/paketo-buildpacks/builder:latest")), any()))
+				.willAnswer(withPulledImage(builderImage));
+		given(docker.image().pull(eq(ImageReference.of("docker.io/cloudfoundry/run:latest")), any()))
+				.willAnswer(withPulledImage(runImage));
+		Builder builder = new Builder(BuildLog.to(out), docker);
+		BuildRequest request = getTestRequest().withBuilder(ImageReference.of("gcr.io/paketo-buildpacks/builder"));
+		builder.build(request);
+		assertThat(out.toString()).contains("Running creator");
+		assertThat(out.toString()).contains("Successfully built image 'docker.io/library/my-application:latest'");
+		ArgumentCaptor<ImageArchive> archive = ArgumentCaptor.forClass(ImageArchive.class);
+		verify(docker.image()).load(archive.capture(), any());
+		verify(docker.image()).remove(archive.getValue().getTag(), true);
+	}
+
 	@Test
 	void buildWhenStackIdDoesNotMatchThrowsException() throws Exception {
 		TestPrintStream out = new TestPrintStream();