JDK8 Image bundles wrong agent jar version again. Regression from 276

[cfraenkel]

Jenkins and plugins versions report

Docker, JDK8 image sha256:38e33ef80be2826094bd4aa01f8886a307e46c3f94a32a29d9f6ab6f2c7f6ae1

What Operating System are you using (both controller, and any agents involved in the problem)?

linux, but doesn't matter

Reproduction steps

docker pull jenkins/inbound-agent:jdk8docker pull jenkins/inbound-agent:jdk8
docker run --rm  jenkins/inbound-agent:jdk8

Expected Results

Container starts and lists help message (and can be used with kubernetes plugin etc.)

Actual Results

Container fails to start because the bundled agent.jar requires a newer java runtime.

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.UnsupportedClassVersionError: hudson/remoting/jnlp/Main has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:473)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:601)

Anything else?

This is the same issue as in #276