Expose classpath, main class, and other variables for reference from an explicit entrypoint

Following on to #579 where it would be useful to be able to somehow reference the computed classpath from within an entrypoint definition.

For example, on Gradle:

jib {
  container {
    entrypoint = ['/opt/startup.sh', '-classpath', classpath.join(':'), '-jvmflags', '"' + jvmFlags.join(' ') + '"']
  }
}

Would we want users to be able to change the classpath? Should we expose the computed main-class?

[chanseokoh]

@remmeier @mfriedenhagen @leonard84 @dtreskunov @OneCricketeer @tellisnz-shift @seanabraham @tomikmar @flobjective

Previously we suggested an extension approach to the issue. With Jib 3.1.1 that we just released, we claim that the issue can be finally resolved with a reasonably good option.

Jib 3.1.1 creates two new JVM argument files inside an image, where they hold the computed classpath and the main class respectively. It will be trivial to retrieve these values from a shell. See the doc for more details.

[briandealwis]

We’re contemplating adding support for surefire-style @{property} resolving (e.g., @{jib.container.mainClass}). These properties would be expanded for the entrypoint, command arguments, and environment variables, allowing something like:

<container>
  <environment>
   <MAINCLASS>@{jib.container.mainClass}</MAINCLASS>
   <CLASSPATH>@{jib.container.classpath}</CLASSPATH>
  </environment>
</container>