add shellscript format checker to CI

ci/validates.gitlab-ci.yml

-validate:shellscripts:
+validate:shellscripts:lint:
   stage: validate
   image: gitlab-registry.mpcdf.mpg.de/tbz/ci_tools:latest
   before_script:
@@ -8,6 +8,16 @@ validate:shellscripts:
     - shellcheck tests/dnf-automatic-mock
   allow_failure: true
 
+validate:shellscripts:format:
+  stage: validate
+  image: gitlab-registry.mpcdf.mpg.de/tbz/ci_tools:latest
+  before_script:
+    - shfmt --version
+  script:
+    - shfmt -i 2 -d script/check_dnf_automatic
+    - shfmt -i 2 -d tests/dnf-automatic-mock
+  allow_failure: true
+
 validate:specfile:
   stage: validate
   before_script: