Fix CPU Dockerfile: use --extra-index-url instead of --index-url

Problem:
- CPU image build was failing in GitHub Actions
- Error: "moshi was not found in the package registry"
- Same issue as GPU Dockerfile had earlier

Root Cause:
- --index-url REPLACES the default PyPI index
- This prevents moshi (on PyPI) from being found
- Only PyTorch index was accessible

Solution:
- Changed --index-url to --extra-index-url
- This ADDS PyTorch CPU index while keeping PyPI access
- Now uv can find both:
  - torch/torchaudio from PyTorch CPU index
  - moshi from PyPI

Difference:
- --index-url: Replaces PyPI (only PyTorch repo)
- --extra-index-url: Adds PyTorch repo (PyPI + PyTorch)

This matches the fix already applied to main Dockerfile in commit 7d0524a.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: mmaudet

Dockerfile.cpu

@@ -31,12 +31,13 @@ WORKDIR /app
 
 # Install PyTorch CPU version and Moshi together to avoid duplicate downloads
 # CPU version works on both ARM64 (Mac M1/M2/M3/M4/M5) and x86_64
+# Use --extra-index-url (not --index-url) to keep access to PyPI for moshi
 # Note: CPU inference is slower than GPU but works everywhere
 RUN uv pip install --system --break-system-packages \
     torch \
     torchaudio \
     moshi \
-    --index-url https://download.pytorch.org/whl/cpu \
+    --extra-index-url https://download.pytorch.org/whl/cpu \
     && rm -rf /root/.cache/uv
 
 # Install core API dependencies using uv