fix: pin python base image to 3.11.9-slim-bookworm for stable Docker build

Dockerfile

@@ -1,7 +1,7 @@
 # Build: docker build -t meta-env:latest .
 # Run:   docker run -p 7860:7860 meta-env:latest
 
-FROM python:3.11-slim
+FROM python:3.11.9-slim-bookworm
 
 WORKDIR /app
 
@@ -25,4 +25,4 @@ EXPOSE 7860
 HEALTHCHECK --interval=30s --timeout=10s --start-period=10s --retries=3 \
     CMD curl -f http://localhost:${PORT}/health || exit 1
 
-CMD ["sh", "-c", "uvicorn server.app:app --host 0.0.0.0 --port ${PORT}"]
+CMD ["sh", "-c", "uvicorn server.app:app --host 0.0.0.0 --port ${PORT}"]