removed master branch from beta builds

.gitlab-ci.yml

@@ -11,6 +11,8 @@ stages:
 
 build: #This will be executed on every push. It builds an docker image, which is named after the branch, so you can always use the last state of the different branches.
   stage: build
+  except:
+    - master
   script:
     - echo "building for branch $CI_COMMIT_REF_NAME"
     - sudo docker image prune -f #Delete old unused images