fix: mount .build-info as volume instead of relying on COPY

Docker build cache can skip picking up the .build-info file during
COPY. Mounting it as a read-only volume ensures the container always
reads the current host file.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -52,6 +52,7 @@ services:
     volumes:
       - ./logs:/app/logs
       - ./uploads:/app/uploads
+      - ./.build-info:/app/.build-info:ro
     mem_limit: 512m
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8000/health"]