Run as user

The current containers are run with some unfortunate defaults:

• the posgres one switches to user 999:999
• the apertis-qa-report runs as root:root (the real one, as userns are not used by Docker)

This raises important security concerns and is cumbersome to manage.

To avoid that, explicitly set the numeric uid:gid when instantiating both containers, so they get run by non-root and file permissions are consistent both inside and outside of the container.