Install torch and torchvision from the CPU-only index before docling
to avoid pulling CUDA/nvidia/triton dependencies. Update documentation
with measured image sizes (270MB remote, 1.9GB local).
Both root and backend Dockerfiles now use a shared base stage with
two targets: remote (lightweight, ~300MB) and local (full Docling,
~2-3GB). docker-compose uses CONVERSION_MODE to select the target.
