Fail on bootBuildImage with launch script

This commit adds a check to the support code for the Gradle plugin
bootBuildImage task to ensure that the jar file that will be passed
to a builder is readable and has a valid directory. This prevents a
situation where the jar file cannot be read because it is prepended
with a launch script, and the builder does not receive any files to
process.

Notes have also been added to the Gradle plugin documentation to warn
against using a bootJar launchScript configuration and bootBuildImage
together, as well as caveats about launchScript that match the Maven
plugin documentation.

Fixes gh-22223

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

@@ -235,7 +235,6 @@ public class BuildRequest {
 	 * @return a new build request instance
 	 */
 	public static BuildRequest forJarFile(File jarFile) {
-		assertJarFile(jarFile);
 		return forJarFile(ImageReference.forJarFile(jarFile).inTaggedForm(), jarFile);
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/io/ZipFileTarArchive.java

@@ -35,6 +35,7 @@ import org.springframework.util.StreamUtils;
  * Adapter class to convert a ZIP file to a {@link TarArchive}.
  *
  * @author Phillip Webb
+ * @author Scott Frederick
  * @since 2.3.0
  */
 public class ZipFileTarArchive implements TarArchive {
@@ -54,6 +55,7 @@ public class ZipFileTarArchive implements TarArchive {
 	public ZipFileTarArchive(File zip, Owner owner) {
 		Assert.notNull(zip, "Zip must not be null");
 		Assert.notNull(owner, "Owner must not be null");
+		assertArchiveHasEntries(zip);
 		this.zip = zip;
 		this.owner = owner;
 	}
@@ -72,6 +74,16 @@ public class ZipFileTarArchive implements TarArchive {
 		tar.finish();
 	}
 
+	private void assertArchiveHasEntries(File jarFile) {
+		try (ZipFile zipFile = new ZipFile(jarFile)) {
+			Assert.state(zipFile.getEntries().hasMoreElements(), "File '" + jarFile.toString()
+					+ "' is not compatible with buildpacks; ensure jar file is valid and launch script is not enabled");
+		}
+		catch (IOException ex) {
+			throw new IllegalStateException("File is not readable", ex);
+		}
+	}
+
 	private void copy(ZipArchiveEntry zipEntry, InputStream zip, TarArchiveOutputStream tar) throws IOException {
 		TarArchiveEntry tarEntry = convert(zipEntry);
 		tar.putArchiveEntry(tarEntry);