Use Docker VHS image for reliable demo GIF rendering

Author: Rohan5commit

.github/workflows/generate-demo.yml

@@ -20,10 +20,9 @@ jobs:
           python -m pip install --upgrade pip
           pip install -e .
 
-      - name: Render VHS demo
-        uses: charmbracelet/vhs-action@v2
-        with:
-          path: .github/demo.tape
+      - name: Render VHS demo (Docker)
+        run: |
+          docker run --rm -v "$PWD":/vhs ghcr.io/charmbracelet/vhs .github/demo.tape
 
       - name: Upload demo artifact
         uses: actions/upload-artifact@v4