Add CI docker linter for nk20 Dockerfile

.gitlab-ci.yml

@@ -42,6 +42,13 @@ ansible-linter:
   image: otthorn/nk20_ci_ansiblelint
   script: ansible-lint ansible/
 
+# Docker linter
+docker-linter:
+  stage: quality-assurance
+  image: hadolint/hadolint
+  script:
+    - hadolint Dockerfile
+
 # Compile documentation
 documentation:
   stage: docs