added linting to ci

.gitlab-ci.yml

 stages:
+  - lint
   - build-image
   - build
   - test
 
+lint:
+  stage: lint
+  tags:
+    - conda, dkrz
+  script:
+    - pre-commit run --all-files
+  rules:
+    - !reference [.default_rules, rules
+
 build:
   stage: build-image
   image: