Security: Container runs as root user by default

## Description

It would be better to have the image run as a non-root user which can also be configurable by the user (just like in [docker-geoserver](https://github.com/geosolutions-it/docker-geoserver/blob/master/Dockerfile#L64-L66)).

**What kind of improvement you want to add?** (check one with "x", remove the others)
- [x] Build related changes
