Support alternative media type format

Update `ExportedImageTar` media type detection to support `tar+gzip`
as well as `tar.gzip`. Recent updates to Docker Desktop appear to have
changed the format.

Fixes gh-43126

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/docker/ExportedImageTarTests.java

@@ -38,7 +38,7 @@ class ExportedImageTarTests {
 	@ParameterizedTest
 	@ValueSource(strings = { "export-docker-desktop.tar", "export-docker-desktop-containerd.tar",
 			"export-docker-desktop-containerd-manifest-list.tar", "export-docker-engine.tar", "export-podman.tar",
-			"export-docker-desktop-nested-index.tar" })
+			"export-docker-desktop-nested-index.tar", "export-docker-desktop-containerd-alt-mediatype.tar" })
 	void test(String tarFile) throws Exception {
 		ImageReference reference = ImageReference.of("test:latest");
 		try (ExportedImageTar exportedImageTar = new ExportedImageTar(reference,