Use python:3.11-slim-bookworm for explicit base image tag

Avoids potential digest-resolution ambiguity with the floating
python:3.11-slim tag during Docker Hub pulls.

Dockerfile

@@ -16,7 +16,7 @@
 # Hugging Face Spaces: set PORT env var (Spaces uses 7860 by default).
 # 
 
-FROM python:3.11-slim
+FROM python:3.11-slim-bookworm
 
 # System deps
 RUN apt-get update && apt-get install -y --no-install-recommends \