[Docker] Do not run as root

We should not have the processes i nDocke rrun as root (default, UID 0).

The www-data user seems a good choice, as long as our images are based on Debian (in other words: forever).

added Security part::backend part::ci part::packaging type::bug labels

created merge request !544 (merged) to address this issue

mentioned in merge request !544 (merged)

closed with merge request !544 (merged)

mentioned in commit c6f14fe1