Extend the base images with an additional argument that allows us to configure a different Python Package Index than default PyPI.

pip and uv both support config files (pip.conf, uv.toml) and env vars to configure path to config file with PIP_CONFIG_FILE and UV_CONFIG_FILE. pip and uv also have the same lookup precedence: command line argument, then env var, and config file last. This allows us to set a default value in a config file and have users pass a different URL as env var at runtime.

NOTE

Authentication is out of scope for now. The INDEX_URL is passed as a build argument and therefore must not contain any credentials.