Ubuntu 24.04 comes with existing non-root user

[yuvipanda]

Ubuntu 24.04 already comes with a non-root user at 1000:

ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash

Ubuntu 22.04 does not have this, so we have been able to create our own with uid 1000. So repo2docker I think will currently fail if we try to use an ubuntu 24.04 base image.