gitlab-ci: Test the recipes syntax

Trigger a debos dry run to prevent syntax errors in the YAML recipes.

.gitlab-ci.yml

-image: debian:stretch
-
-before_script:
-  - export DEBIAN_FRONTEND=noninteractive
-  - apt-get update
-  - apt-get install --no-install-recommends -y gradle
-
-unit-tests:
+jenkinsfile-unit-tests:
+  image: debian:stretch
   stage: test
+  before_script:
+    - export DEBIAN_FRONTEND=noninteractive
+    - apt-get update
+    - apt-get install --no-install-recommends -y gradle
   script:
     - gradle --no-daemon test
   artifacts:
@@ -15,3 +13,9 @@ unit-tests:
     reports:
       junit:
         - build/test-results/test/TEST-*.xml
+
+debos-dry-run:
+  image: docker-registry.apertis.org/apertis/apertis-v2019pre-image-builder
+  stage: test
+  script:
+    - 'for i in *.yaml ; do debos --print-recipe --dry-run "$i" ; done'