Convert Docker build to multi-stage runtime image

[finbarrtimbers]

Initial implementation done by Codex.

Previously, our image was ~21.5GiB. Now, it's 17.1GiB.

Summary

• introduce a dedicated CUDA devel builder stage to resolve Python dependencies with uv
• copy the prepared virtual environment into a slimmer CUDA runtime stage that installs only runtime packages

Runs:

1. Single GPU: Beaker
2. Tool use: Beaker
3. DPO: Beaker
4. Multi-node GRPO: Beaker

[finbarrtimbers]

Annoyingly DeepSpeed requires NVCC! Closing this until/if we remove DeepSpeed.