Manifests of jars built with Gradle do not have a Build-Jdk-Spec entry

[scottfrederick]

Maven adds a Build-Jdk-Spec attribute to META-INF/MANIFEST.MF when building a jar or war file. Paketo Java buildpacks will detect this manifest attribute and use the value of it to decide the major version of the JVM to install in an OCI image.

The Spring Boot Gradle plugin should add Build-Jdk-Spec to the manifest also, to provide similar benefit when a Gradle-built application is packaged by Paketo buildpacks. Other Cloud Native Buildpack vendors could then choose to use this technique also.

Once this is done, the Spring Boot Maven spring-boot:build-image goal and Gradle plugin bootBuildImage task won't need to set the Paketo-specific BP_JVM_VERSION environment variable to configure the JVM version that buildpacks install.

[philwebb]

We consider this one a bug because the buildpack can use the entry and we'd like to have alignment with Maven and Gradle, however, it feels too risky for 2.x.