Fix Ubuntu 24.04 systemtest Docker user setup

[PranjalManhgaye]

Summary

This fixes the Ubuntu 24.04 systemtest Docker build when the requested host UID/GID already exists inside the base image.

With "ubuntu:24.04", UID/GID 1000 can already be used by the default ubuntu user/group. The previous Dockerfile always tried to create a new precice group and user with those IDs, which failed with:

groupadd: GID '1000' already exists

Changes

The Dockerfile now checks whether the requested group or user already exists:

• If the requested GID already exists, it reuses that group by renaming it to precice.
• If the requested UID already exists, it reuses that user by renaming it to precice.
• If the UID/GID are unused, it creates the precice group/user as before.
• It ensures /home/precice exists and is owned by the requested UID/GID.
• This keeps the later USER precice behavior working while preserving the host UID/GID mapping used by local system tests.
  Fixes System test Docker container for Ubuntu 24.04 fails in groupadd #766.

Both builds ran successfully

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[MakisH]

Thanks for the fix! This is important for developing/debugging locally.

It runs on my system (Ubuntu 22.04, UID/GID=1000), and the changes make sense to me.

Since I need to get back into context, could you please elaborate on why this is an issue with the Ubuntu 24.04 builds, but not with the 22.04?

[PranjalManhgaye]

Thanks @MakisH for a check , the difference seems to come from the base images themselves. " ubuntu:24.04" already includes a default "ubuntu" user/group with UID/GID 1000, while " ubuntu:22.04" does not.
I checked this with :
docker run --rm ubuntu:24.04 bash -lc 'getent passwd 1000; getent group 1000'
which returns :

For " ubuntu:22.04", the same command does not return a UID/GID 1000 entry.
So the previous Dockerfile worked on 22.04 because groupadd -g 1000 precice and useradd -u 1000 ... had no collision there. on 24.04, the same commands collide with the existing ubuntu user/group before the precice user can be created , This patch handles both cases : 1) if the requested UID/GID already exists, it reuses that entry for precice; otherwise 2) it keeps the previous behavior and creates the precice user/group normally.