fix: use docker cp instead of volume mount for nginx deploy

The act runner's Docker volume mount path doesn't resolve to a
valid host path, causing 403 Forbidden from nginx. Copy files
directly into the container instead.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/deploy.yaml

@@ -31,11 +31,10 @@ jobs:
 
       - name: Deploy to nginx
         run: |
-          chmod -R o+rX dist
           docker rm -f slides-nginx 2>/dev/null || true
           docker run -d \
             --name slides-nginx \
             --restart unless-stopped \
             -p 8080:80 \
-            -v ${{ github.workspace }}/dist:/usr/share/nginx/html:ro \
             nginx:alpine
+          docker cp dist/. slides-nginx:/usr/share/nginx/html/