Update maven example to handle "cannot create resource output directory" error

[noelmcloughlin]

Describe the bug

Maven-resources-plugin cannot create resource output directory (/home/build/target/) because maven lacks permissions to remove subdirectories under /home/build/ (see similar issue).

To Reproduce

1. Open terminal as some random user
2. Follow https://edu.chainguard.dev/chainguard/chainguard-images/reference/maven#springboot until you execute this command:

docker run --platform=linux/amd64 --rm -v ${PWD}:/home/build cgr.dev/chainguard/maven:openjdk-17 clean install

3. See error from maven-resources-plugin

Expected behavior

This worked for me:

chmod o+w ../demo
docker run --platform=linux/amd64 --rm -v ${PWD}:/home/build cgr.dev/chainguard/maven:openjdk-17 clean install
chmod o-w ../demo     # optional, its just demo

[ltagliaferri]

Thanks for flagging this, I copied this over to the Images repo, where the readmes live. chainguard-images/images#2624