Add runImage option for image building

This commit adds a runImage property to the Maven plugin build-image
goal and the Gradle bootBuildImage task. The property allows the user
to override the run image reference provided in the builder metadata
with an alternate run image. The runImage property can be specified
in the build file or on the command line.

Fixes gh-21534

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootBuildImage.java

@@ -61,6 +61,8 @@ public class BootBuildImage extends DefaultTask {
 
 	private String builder;
 
+	private String runImage;
+
 	private Map<String, String> environment = new HashMap<>();
 
 	private boolean cleanCache;
@@ -133,6 +135,26 @@ public class BootBuildImage extends DefaultTask {
 		this.builder = builder;
 	}
 
+	/**
+	 * Returns the run image that will be included in the built image. When {@code null},
+	 * the run image bundled with the builder will be used.
+	 * @return the run image
+	 */
+	@Input
+	@Optional
+	public String getRunImage() {
+		return this.runImage;
+	}
+
+	/**
+	 * Sets the run image that will be included in the built image.
+	 * @param runImage the run image
+	 */
+	@Option(option = "runImage", description = "The name of the run image to use")
+	public void setRunImage(String runImage) {
+		this.runImage = runImage;
+	}
+
 	/**
 	 * Returns the environment that will be used when building the image.
 	 * @return the environment
@@ -228,6 +250,7 @@ public class BootBuildImage extends DefaultTask {
 
 	private BuildRequest customize(BuildRequest request) {
 		request = customizeBuilder(request);
+		request = customizeRunImage(request);
 		request = customizeEnvironment(request);
 		request = customizeCreator(request);
 		request = request.withCleanCache(this.cleanCache);
@@ -242,6 +265,13 @@ public class BootBuildImage extends DefaultTask {
 		return request;
 	}
 
+	private BuildRequest customizeRunImage(BuildRequest request) {
+		if (StringUtils.hasText(this.runImage)) {
+			return request.withRunImage(ImageReference.of(this.runImage));
+		}
+		return request;
+	}
+
 	private BuildRequest customizeEnvironment(BuildRequest request) {
 		if (this.environment != null && !this.environment.isEmpty()) {
 			request = request.withEnv(this.environment);