When the configuration is declared on an execution rather than on the plugin as a whole, it only affects that specific execution. Crucially, it means that it you run ./mvnw spring-boot:build-image the configuration has no effect. We should perhaps show the configuration being applied to the whole plugin, or at least document the subtleties of Maven's behaviour here.
Comment From: snicoll
I think this is a template I've used based on our repackage execution of the repackage goal in our parent pom. Given we have no such constraint, we should really document things differently. I'll update the doc to that effect. Thanks for raising it!