[Maintenance] reuse use with uid=1000 from distro

[GioF71]

Starting with lunar, the ubuntu docker image presents a user named "ubuntu" with uid set to 1000. Exactly the uid we need when using user mode, unless the local user id which runs the image does not have the uid 1000 (which is usually the default id for the first user).

The fact that the user is named 'ubuntu' should not be a problem. So instead of deleting the user in the dockerfile, we might reuse it in the entrypoint script. In this case, we cannot determine the user name (and so maybe the user directory path), but we should use the username that is set by the distro maintainer.

This is not a major issue, but let's see if we can have a cleaner build and run strategy.