Fix PyTorch install URL: use --extra-index-url for CPU wheels

The -f torch_stable.html URL didn't have the correct CPU wheels.
Switch to --extra-index-url https://download.pytorch.org/whl/cpu
which is the official way to install PyTorch CPU-only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.10-slim
 
-# System dependencies
+# System dependencies (gcc/g++ needed for pycocotools)
 RUN apt-get update && apt-get install -y \
     libgl1-mesa-glx \
     libglib2.0-0 \
@@ -10,16 +10,16 @@ RUN apt-get update && apt-get install -y \
 
 WORKDIR /app
 
-# Install PyTorch CPU first
+# Install PyTorch CPU first (use --extra-index-url for CPU wheels)
 RUN pip install --no-cache-dir \
     torch==1.13.1+cpu \
     torchvision==0.14.1+cpu \
-    -f https://download.pytorch.org/whl/cpu/torch_stable.html
+    --extra-index-url https://download.pytorch.org/whl/cpu
 
-# Install mmcv (lite version - no C++ ops needed)
+# Install mmcv (lite version - pure Python, no C++ compilation)
 RUN pip install --no-cache-dir mmcv==1.7.2
 
-# Copy and run mmcv.ops shim setup
+# Create mmcv.ops shim using torchvision
 COPY setup_mmcv_shim.py .
 RUN python setup_mmcv_shim.py