Expose CMAKE_CUDA_ARCHITECTURES as a Docker build ARG so users can
customize CUDA architectures via --build-arg without editing the
Dockerfile.
- convert hardcoded ENV to ARG with default, feeding into ENV
- replace silent fallback defaults (:-) in scripts with :? guards
to fail fast if the env var is missing
- add usage example to Dockerfile header
Follow up to: #624https://claude.ai/code/session_01EWiUe7jNABX7Uz95dUGJqK
Co-authored-by: Claude <noreply@anthropic.com>
Benson Wong