Fix documentation of configuring extraDirectories for Gradle plugin

[KENNYSOFT]

Current documentation:

https://github.com/GoogleContainerTools/jib/blob/c419354aa42c4c8a2a32f93c1524d0b31b598979/docs/faq.md#i-need-to-add-files-generated-during-the-build-process-to-a-custom-directory-on-the-image

The same can be accomplished in Gradle by using a Copy task. In your build.gradle:

jib.extraDirectories = file('build/extra-directory')

does not meet the syntax:

https://github.com/GoogleContainerTools/jib/blob/b7bdec593cc4b8614c9a66e2487b2a2ad80f6aed/jib-gradle-plugin#adding-arbitrary-files-to-the-image

You can configure different directories by using the jib.extraDirectories.paths parameter in your build.gradle:

jib {
  // Copies files from 'src/main/custom-extra-dir' and '/home/user/jib-extras' instead of 'src/main/jib'
  extraDirectories.paths = ['src/main/custom-extra-dir', '/home/user/jib-extras']
}

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[KENNYSOFT]

+ docker kill
"docker kill" requires at least 1 argument.
See 'docker kill --help'.

Usage: docker kill [OPTIONS] CONTAINER [CONTAINER...]

Kill one or more running containers
+ true
+ '[' MACOS_EXTERNAL = MACOS_EXTERNAL ']'
+ osascript -e 'quit app "Docker"'
+ open -a Docker
Unable to find application named 'Docker'

[ID: 7921967] Build finished after 45 secs, exit value: 1

Have no idea why 'kokoro-macos' check fails 🤔

[zhumin8]

Thanks for catching this.
'kokoro-macos' check fail is related to an internal infra error. We'll fix and rerun this check.

[zhumin8]

Thanks for catching this and making our FAQ more consistent!
jib.extraDirectories = file('build/extra-directory') is one of the older syntax shown here, that is later deprecated in #1671 and removed in #2108.