use own docker image for linter

.gitlab-ci.yml

-image: debian:latest
+image: registry.git.cccv.de/infra/uffd/docker-images/stretch
 
 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
   DEBIAN_FRONTEND: noninteractive 
 
 before_script:
-  - apt update
-  - apt -y -qq -o Dpkg::Use-Pty=0 install python3 sqlite3 locales-all git python3-flask python3-coverage python3-flask-sqlalchemy python3-pip python-ldap3
   - python3 -V
+  - lsb_release -a
   - uname -a
 
 linter: