Allow digest builder references

Update `BuildRequest` to also allow digest based references.

Closes gh-21879

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

@@ -84,8 +84,9 @@ public class BuildRequest {
 	 */
 	public BuildRequest withBuilder(ImageReference builder) {
 		Assert.notNull(builder, "Builder must not be null");
-		return new BuildRequest(this.name, this.applicationContent, builder.inTaggedForm(), this.creator, this.env,
-				this.cleanCache, this.verboseLogging);
+		builder = (builder.getDigest() != null) ? builder : builder.inTaggedForm();
+		return new BuildRequest(this.name, this.applicationContent, builder, this.creator, this.env, this.cleanCache,
+				this.verboseLogging);
 	}
 
 	/**

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

@@ -98,6 +98,14 @@ public class BuildRequestTests {
 		assertThat(request.getBuilder().toString()).isEqualTo("docker.io/spring/builder:latest");
 	}
 
+	@Test
+	void withBuilderWhenHasDigestUpdatesBuilder() throws IOException {
+		BuildRequest request = BuildRequest.forJarFile(writeTestJarFile("my-app-0.0.1.jar")).withBuilder(ImageReference
+				.of("spring/builder:@sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d"));
+		assertThat(request.getBuilder().toString()).isEqualTo(
+				"docker.io/spring/builder:@sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d");
+	}
+
 	@Test
 	void withCreatorUpdatesCreator() throws IOException {
 		BuildRequest request = BuildRequest.forJarFile(writeTestJarFile("my-app-0.0.1.jar"));