fix: add non-free apt sources for nvidia-cuda-toolkit on Bookworm

[harshmathurx]

The python:3.12-slim-bookworm base image only has main and non-free-firmware components enabled. nvidia-cuda-toolkit lives in non-free, and some of its install-time dependencies resolve from contrib, so the GPU Docker build needs both components enabled before installing it.

This PR adds deb http://deb.debian.org/debian bookworm contrib non-free to sources.list before the GPU install block so ENABLE_GPU=true builds don't fail with:

Package nvidia-cuda-toolkit has no installation candidate

Also fixes a stray trailing space after apt-get clean \.

Fixes #2020.

Validation:

• Simulated Bookworm apt sources matching the slim image (main non-free-firmware + this added source).
• apt-cache policy nvidia-cuda-toolkit finds candidate 11.8.89~11.8.0-5~deb12u1.
• apt-get install -s --no-install-recommends nvidia-cuda-toolkit resolves successfully with contrib non-free; it fails with non-free alone due missing dependencies.

[harshmathurx]

@ntohidi gentle ping for review when you have a moment. I have retargeted this to develop and updated the apt source to contrib non-free after validating that non-free alone makes nvidia-cuda-toolkit visible but still leaves dependency resolution broken in a Bookworm slim-style apt simulation.