Add a stage to clean compiled classes.

.gitlab-ci.yml

 image: git.fortiss.org:5001/af3/maven-releng:latest
 
 stages:
+  - clean
   - build
   - test
   - deploy
@@ -16,6 +17,17 @@ variables:
   MAVEN_TEST_OPTS: "-Dbuild.tests=true"
   MAVEN_BUILD_DIR: "-Duser.dir=/builds/$CI_PROJECT_PATH"
 
+clean:
+  stage: clean
+  cache:
+    key: Default
+  rules:
+    - if: '$BUILD_TYPE == "clean"' # This rule will be evaluated
+      when: on_success
+  before_script:
+    - git submodule update $GIT_SUBMODULE_OPTS
+  script:
+    - mvn $MAVEN_CLI_OPTS $MAVEN_BUILD_OPTS clean
 
 build:
   stage: build