fix: use Debian Bullseye base image (has OpenJDK 11 for SPICE Nashorn engine)

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.10-slim-bookworm
+FROM python:3.10-slim-bullseye
 
 # Install OpenJDK 11 (SPICE requires Nashorn JS engine, removed in Java 15+)
 RUN apt-get update && apt-get install -y --no-install-recommends \