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-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-buildsImageWithBuildpackFromBuilder.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	buildpacks = [ "paketo-buildpacks/java" ]
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-buildsImageWithBuildpackFromDirectory.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	buildpacks = [ "file://${projectDir}/buildpack/hello-world" ]
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-buildsImageWithBuildpackFromImage.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	buildpacks = [ "gcr.io/paketo-buildpacks/java:latest" ]
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-buildsImageWithBuildpackFromTarGzip.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	buildpacks = [ "file://${projectDir}/hello-world.tgz" ]
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-failsWithBuildpackNotInBuilder.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
+
+bootBuildImage {
+	buildpacks = [ "urn:cnb:builder:example/does-not-exist:0.0.1" ]
+}