fix(docker): use python:3.11-slim base for >=3.9 requirement

The Publish Docker image workflow broke on master after the minimum
Python bump to 3.9: the base image dptechnology/dflow:latest ships
Python 3.8.20, so `pip install .` aborts with "Package 'dpgen2'
requires a different Python: 3.8.20 not in '>=3.9'".

Switch the base to python:3.11-slim. pydflow is a declared dependency
of dpgen2, so it is installed by `pip install .` and need not be baked
into the base image. All install-time dependencies ship manylinux
wheels (no source build), so the slim image is sufficient.

Author: Han Wang

Dockerfile

@@ -1,4 +1,4 @@
-FROM dptechnology/dflow:latest
+FROM python:3.11-slim
 
 WORKDIR /data/dpgen2
 COPY ./ ./