Create a dedicated app user and group in multistage.Dockerfile - #32

I agree that we should be consistent with the user and group setting, though with this change, i'm getting an error: PermissionError: Permission denied (os error 13) about ["/etc/ssl/private"]

@konstin Thank you for reviewing.
The error message didn't arise in my environments (Lima on macOS, Docker rootless mode on Linux, and Kaniko on Linux).
What environment did you build Dockerfile on?

A normal docker installation (with root) on Ubuntu 24.04:

$ docker --version
Docker version 24.0.7, build 24.0.7-0ubuntu4.1

docker build -f multistage.Dockerfile . -t uv-multistage
docker run --rm -it --publish 8000:8000 uv-multistage

I haven't tried container build on the same environment yet, but CI build job on ubuntu-latest succeeded.
https://github.com/toshikish/uv-docker-example/actions/runs/12113065492/job/33767462978#step:5:210

The build succeeds, it produces this error at runtime: It tries to read the SSL certificates, but they are owned by root so it can't access them.