Jib: Adds init script to use with any Gradle project.

[coollog]

Fixes #63

The init script auto-applies jib-gradle-plugin to the Gradle project.
Also fixes the caching for the jib-gradle BuildTemplate.

/cc @loosebazooka
/cc @saturnism

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In response to this:

Fixes #63

The init script auto-applies jib-gradle-plugin to the Gradle project.
Also fixes the caching for the jib-gradle BuildTemplate.

/cc loosebazooka
/cc saturnism

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[imjasonh]

Even though it's annoying to have two copies, it might be helpful to be explicit about the steps to speed up Gradle builds specifically. I'm thinking if I'm a heavy Gradle user and doesn't know anything about Maven, I don't want to have to figure out which parts of the Maven-specific "speeding up builds" doc apply to me and which don't.

Let me know if that's a silly thought, and that Gradle developers would necessarily know enough about Maven to figure it out, I don't know the Javas.

[imjasonh]

This might indicate that we want a jib-gradle builder image that includes these files (and any future ones), without having to squeeze them into the build template.

If we go that route, my personal preference would be to base that image FROM gradle and not rely on the poorly-supported gcr.io/cloud-builders/gradle image. (Same with maven, tbh). That can happen in a later CL though if you'd prefer.

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