Fix Docker build by adding USER root before useradd commands

The new Python 3.14 base image (registry.gitlab.syncad.com/hive/wax/ci-base-image)
runs as a non-root user by default, causing useradd to fail with
"Permission denied" and "cannot lock /etc/passwd".

Add USER root directive at the start of both preconfigured_base_image
and python_installer stages to ensure root privileges for user creation.