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

@@ -126,6 +126,15 @@ class BootBuildImageIntegrationTests {
 		}
 	}
 
+	@TestTemplate
+	void failsWithLaunchScript() {
+		writeMainClass();
+		writeLongNameResource();
+		BuildResult result = this.gradleBuild.buildAndFail("bootBuildImage");
+		assertThat(result.task(":bootBuildImage").getOutcome()).isEqualTo(TaskOutcome.FAILED);
+		assertThat(result.getOutput()).contains("not compatible with buildpacks");
+	}
+
 	@TestTemplate
 	void failsWithBuilderError() {
 		writeMainClass();