Start pipeline only when committing to master

.gitlab-ci.yml

@@ -9,12 +9,13 @@ variables:
 build-p2:
   stage: build
   tags: 
-    - docker # use 
+    - docker
   image: maven:3.6.3-adoptopenjdk-15
   script:
     - mvn --version
     - mvn clean install
-  when: manual
+  only:
+    - master
   artifacts:
     paths:
       - de.hft-stuttgart.indriya.p2site/target/repository/