This commit adds a check to the support code for the Gradle plugin
bootBuildImage task to ensure that the jar file that will be passed
to a builder is readable and has a valid directory. This prevents a
situation where the jar file cannot be read because it is prepended
with a launch script, and the builder does not receive any files to
process.

Notes have also been added to the Gradle plugin documentation to warn
against using a bootJar launchScript configuration and bootBuildImage
together, as well as caveats about launchScript that match the Maven
plugin documentation.

Fixes gh-22223