build container: ensure proper permissions are used

[rgerhards]

Right now, the docker container by default uses root user. We would like to switch this to current user, if possible. Among others, I have asked this here:

https://forums.docker.com/t/automatically-run-container-under-current-user/45289

If it is not possible with Docker natively, we should consider providing a run script, but that's mood because the whole point of the container is that we do not need to install anything upfront. So maybe the doc needs to fix that.

Also, we need to check if the volumes are created with the right permissions.

[rgerhards]

so far, it looks like this in doable.