ci: utilise une image slim de debian

.gitlab-ci.yml

 variables:
   DEBIAN_FRONTEND: noninteractive
+  DOCKER_IMAGE: python:3.11-slim-bookworm
 
 stages:
   - lint
 
 lint:
   stage: lint
-  image: python:3.11-bookworm
+  image: $DOCKER_IMAGE
   script:
-  - pip install virtualenv
-  - virtualenv --python=/usr/bin/python3 .
+  - pip3 install virtualenv
+  - virtualenv .
   - source bin/activate
   - pip3 install -r requirements.txt
   - bin/pycodestyle --first --show-source --ignore=E501 *.py