Add JavaContainerBuilder to jib core

[TadCordle]

Fixes #1212.

Adds a JavaContainerBuilder class that can be used to build Java-specific containers. Example usage:

JavaContainerBuilder.fromDistroless()
  .addClasses(Paths.get("application/classes"))
  .addResources(Paths.get("application/resources"))
  .addDependencies(
      Paths.get("application/dependencies/dependency.jar"),
      Paths.get("application/dependencies/dependency2.jar"),
      Paths.get("application/dependencies/dependency3-SNAPSHOT.jar)) // Separates snapshots to their own layer
  .addToClasspath(Paths.get("extra/classpath/dir")) // Adds to extra layer, but also adds new directory to classpath
  .setJvmFlags("-Xms512m", "-Xdebug")
  .setMainClass("HelloWorld")
  .toContainerBuilder()
  .containerize(...);

Potential followup issues:

• Add a WarContainerBuilder for WAR-specific case, or adjust JavaContainerBuilder to support WARs
• Use JavaContainerBuilder in Gradle/MavenLayerConfigurations

[chanseokoh]

Sorry if this has been discussed, but for all of these, what if there are duplicate file names?

[chanseokoh]

Sorry if this has been discussed, but for all of these, what if there are duplicate file names?

I mean I know what happens (only the last file will win and survive), e.g.,

  .addToClasspath("/loser/messages.properties",
                  "/winner/messages.properties")

so my question is if this is what we really intended, and if so, I think we should explain it in the Javadocs.

[TadCordle]

@chanseokoh I can't remember where it was discussed, but I think this is intended behavior, and you are correct that I did forget to mention it in the javadocs. Will fix.

[coollog]

This LGTM - let's follow up this with having our plugins use this new API and make sure everything works the same as before.

[chanseokoh]

Since @coollog said LGTM, let's just try. We can iterate if needed.