Switching default to include layers index for fat jar

[mbhave]

No description provided.

[philwebb]

We'd need a Gradle API to switch off support. Since this is quite late for 2.3, we'll reassess in 2.4

[bclozel]

We also need to decide whether we should include the layertools classes to the fat jar, or just the idx file.

[wilkinsona]

Looking at #22195 caused me to notice that there are a few places in the Gradle plugin, javadoc for example, where it reads as if layers are disabled by default. Re-opening so that we can address those.

[bclozel]

It seems that both layers.idx and layertools are included in the fat jar by default now. You can only exclude the layertools from the jar explicitly.

I know that layertools isn't that big, but in this case should this be included with the launcher? I'm a bit confused by this situation.

[mbhave]

I thought it made sense to include layertools by default if layers.idx was present by default as well. I'm not sure what you mean by

but in this case should this be included with the launcher?

but we can discuss this on the next call.

[snicoll]

I'm not sure what you mean by

The use case we want to promote is CNB and our build image build integration. This does not require layertools so that seems a bit strange to include it by default.

This would break users creating docker image and relying on that when upgrading to 2.3 but they'd have to change the enabled flag to includeLayerTools. That doesn't sound too bad to me.

[snicoll]

We've discussed this and we concluded that offering a good out-of-the-box experience for both CNB users (via our build image task in the Maven and Gradle plugins) as well as those who prefer to craft their Dockerfile themselves is important. Advanced users can opt-out for layers, the layertools or both via configuration.