Add buildpack option for image building

This commit adds configuration to the Maven and Gradle plugins to
allow a list of buildpacks to be provided to the image building
goal and task.

Fixes gh-21722

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.maven;
 
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.function.Function;
 
@@ -26,6 +27,7 @@ import org.apache.maven.artifact.versioning.VersionRange;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.boot.buildpack.platform.build.BuildRequest;
+import org.springframework.boot.buildpack.platform.build.BuildpackReference;
 import org.springframework.boot.buildpack.platform.build.PullPolicy;
 import org.springframework.boot.buildpack.platform.io.Owner;
 import org.springframework.boot.buildpack.platform.io.TarArchive;
@@ -65,6 +67,7 @@ class ImageTests {
 		assertThat(request.isCleanCache()).isFalse();
 		assertThat(request.isVerboseLogging()).isFalse();
 		assertThat(request.getPullPolicy()).isEqualTo(PullPolicy.ALWAYS);
+		assertThat(request.getBuildpacks()).isEmpty();
 	}
 
 	@Test
@@ -123,6 +126,15 @@ class ImageTests {
 		assertThat(request.isPublish()).isTrue();
 	}
 
+	@Test
+	void getBuildRequestWhenHasBuildpacksUsesBuildpacks() {
+		Image image = new Image();
+		image.buildpacks = Arrays.asList("example/buildpack1@0.0.1", "example/buildpack2@0.0.2");
+		BuildRequest request = image.getBuildRequest(createArtifact(), mockApplicationContent());
+		assertThat(request.getBuildpacks()).containsExactly(BuildpackReference.of("example/buildpack1@0.0.1"),
+				BuildpackReference.of("example/buildpack2@0.0.2"));
+	}
+
 	private Artifact createArtifact() {
 		return new DefaultArtifact("com.example", "my-app", VersionRange.createFromVersion("0.0.1-SNAPSHOT"), "compile",
 				"jar", null, new DefaultArtifactHandler());