Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -22,10 +22,15 @@ doxygen:
   tags:
     - documentation
   before_script:
-    - apt-get update && apt-get install doxygen
+    - apt-get update && apt-get install -y doxygen
   script:
     - git clone https://gitlab-ci-token:${CI_JOB_TOKEN}@git.rwth-aachen.de/unicado/libraries
     - doxygen libraries/aircraftGeometry2/doc/Doxyfile
+  rules:
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+      when: on_success
+    - if: '$CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH'
+      when: manual
 
 pages:
   stage: deploy