ci: build-doc now doesnt run on tags

.gitlab-ci.yml

@@ -16,15 +16,17 @@ docker-image:
     - if: $CI_COMMIT_TAG
 
 build-doc:
-    image: gitlab.liu.se:5000/phd-courses/multiple-target-tracking
-    stage: build
-    script:
-        - pdoc --html src
-        - mkdir .public
-        - mv html/src/* .public/
-    artifacts:
-        paths:
-            - .public
+  image: gitlab.liu.se:5000/phd-courses/multiple-target-tracking
+  stage: build
+  script:
+    - pdoc --html src
+    - mkdir .public
+    - mv html/src/* .public/
+  artifacts:
+    paths:
+      - .public
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
 
 pages:
   stage: deploy