In shorts customization is not possible with previous blog and need more modification and its complicated. All configs of hivemq and extensions hard coded in docker image.
But why we are getting ReadOnly FileSystem error while mounting volume in same path.
chown: changing ownership of ‘/opt/hivemq/conf/config.xml’: Read-only file system
So we need to go with newer blog which you have share for better customization and automation of hivemq deployment on k8s cluster.
Please go and make a changes in latest hivemq version and let us know. It will be good if ReadyOnly issue resolve.