add reuse ci step

.gitlab-ci.yml

@@ -36,6 +36,15 @@ test:
             junit:
                 - junit.xml
 
+reuse:
+    stage: test
+    image:
+        name: fsfe/reuse:latest
+        entrypoint: [""]
+    before_script: []
+    script:
+        - reuse lint
+
 build:
     stage: build
     script: