use geonetwork.dir environment variable over custom data_dir

Current image has a docker-only param `DATA_DIR` which sets geonetwork.dir as a java environment parameter in a deploy script

https://github.com/geonetwork/docker-geonetwork/blob/9fcf08514e1cbcd6b4f48a1d6acfac9fd23afa2d/3.10.0/docker-entrypoint.sh#L9

In stead would be better to use the mechanism available in geonetwork to set the data directory with environment variable by using
```
docker run -e genoetwork.dir=/mnt/gn
```

Java environment variable takes preference over system environment variable, so you can not override data_dir by geonetwork.dir (data_dir takes default value if not set)

Start with docker-entrypoint.sh around line 9 and review how DATA_DIR is passed to GeoNetwork. Compare that behavior with the documented geonetwork.dir environment-variable mechanism and verify using a Docker run that the environment variable controls the data directory without being overridden by the script.

Written by the indexing model from the issue text.