build docker image only for particular branches

.gitlab-ci.yml

@@ -3,6 +3,10 @@ stages:
 
 build-webssh-docker:
   stage: build-webssh-docker
+  only:
+    - master
+    - rc3.0
+    - rc3.1 
   image: docker:19.03.1
   services:
     - docker:19.03.1-dind