[Hardening] Rootless base-image 

This image has to run as root (see #9 #10).
I would like to hardening this base-image according to the principle of least privilege like bitnami images. 
This could be a step towards this kind of issues : 
https://github.com/osixia/docker-openldap/issues/544
https://github.com/osixia/docker-openldap/issues/398

Any help or advice is welcome. 

