Respect image platform when exporting buildpack layers

Ensure the builder propagates the requested image platform to
DockerApi.exportLayers so Docker 1.41+ saves layers for the pulled
architecture rather than the host default. Add platform-aware tests
issue: #46665

Signed-off-by: hojooo <ghwn5833@gmail.com>

Author: hojooo

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

@@ -355,7 +355,7 @@ public Image fetchImage(ImageReference reference, ImageType imageType) throws IO
 		@Override
 		public void exportImageLayers(ImageReference reference, IOBiConsumer<String, TarArchive> exports)
 				throws IOException {
-			Builder.this.docker.image().exportLayers(reference, exports);
+			Builder.this.docker.image().exportLayers(reference, this.imageFetcher.defaultPlatform, exports);
 		}
 
 	}

buildpack/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/DockerApi.java

@@ -311,9 +311,24 @@ public void load(ImageArchive archive, UpdateListener<LoadImageUpdateEvent> list
 		 */
 		public void exportLayers(ImageReference reference, IOBiConsumer<String, TarArchive> exports)
 				throws IOException {
+			exportLayers(reference, null, exports);
+		}
+
+		/**
+		 * Export the layers of an image as {@link TarArchive TarArchives}.
+		 * @param reference the reference to export
+		 * @param platform the platform (os/architecture/variant) of the image to export
+		 * @param exports a consumer to receive the layers (contents can only be accessed
+		 * during the callback)
+		 * @throws IOException on IO error
+		 */
+		public void exportLayers(ImageReference reference, @Nullable ImagePlatform platform,
+				IOBiConsumer<String, TarArchive> exports) throws IOException {
 			Assert.notNull(reference, "'reference' must not be null");
 			Assert.notNull(exports, "'exports' must not be null");
-			URI uri = buildUrl("/images/" + reference + "/get");
+			URI uri = (platform != null)
+					? buildUrl(PLATFORM_API_VERSION, "/images/" + reference + "/get", "platform", platform)
+					: buildUrl("/images/" + reference + "/get");
 			try (Response response = http().get(uri)) {
 				try (ExportedImageTar exportedImageTar = new ExportedImageTar(reference, response.getContent())) {
 					exportedImageTar.exportLayers(exports);

buildpack/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/docker/DockerApiTests.java

@@ -447,6 +447,38 @@ void exportLayersWithSymlinksExportsLayerTars() throws Exception {
 				.containsExactly("/cnb/stack.toml");
 		}
 
+		@Test
+		void exportLayersWithPlatformExportsLayerTars() throws Exception {
+			ImageReference reference = ImageReference.of("docker.io/paketobuildpacks/builder:base");
+			ImagePlatform platform = ImagePlatform.of("linux/amd64");
+			URI exportUri = new URI(
+					PLATFORM_IMAGES_URL + "/docker.io/paketobuildpacks/builder:base/get?platform=linux%2Famd64");
+			given(DockerApiTests.this.http.head(eq(new URI(PING_URL))))
+				.willReturn(responseWithHeaders(new BasicHeader(DockerApi.API_VERSION_HEADER_NAME, "1.41")));
+			given(DockerApiTests.this.http.get(exportUri)).willReturn(responseOf("export.tar"));
+			MultiValueMap<String, String> contents = new LinkedMultiValueMap<>();
+			this.api.exportLayers(reference, platform, (name, archive) -> {
+				ByteArrayOutputStream out = new ByteArrayOutputStream();
+				archive.writeTo(out);
+				try (TarArchiveInputStream in = new TarArchiveInputStream(
+						new ByteArrayInputStream(out.toByteArray()))) {
+					TarArchiveEntry entry = in.getNextEntry();
+					while (entry != null) {
+						contents.add(name, entry.getName());
+						entry = in.getNextEntry();
+					}
+				}
+			});
+			assertThat(contents).hasSize(3)
+				.containsKeys("70bb7a3115f3d5c01099852112c7e05bf593789e510468edb06b6a9a11fa3b73/layer.tar",
+						"74a9a50ece13c025cf10e9110d9ddc86c995079c34e2a22a28d1a3d523222c6e/layer.tar",
+						"a69532b5b92bb891fbd9fa1a6b3af9087ea7050255f59ba61a796f8555ecd783/layer.tar");
+			assertThat(contents.get("70bb7a3115f3d5c01099852112c7e05bf593789e510468edb06b6a9a11fa3b73/layer.tar"))
+				.containsExactly("/cnb/order.toml");
+			assertThat(contents.get("74a9a50ece13c025cf10e9110d9ddc86c995079c34e2a22a28d1a3d523222c6e/layer.tar"))
+				.containsExactly("/cnb/stack.toml");
+		}
+
 		@Test
 		void tagWhenReferenceIsNullThrowsException() {
 			ImageReference tag = ImageReference.of("localhost:5000/ubuntu");