make automatic staging deployments

Author: mordamax

.gitlab-ci.yml

@@ -35,6 +35,7 @@ default:
 
 .staging-refs:                     &staging-refs
   rules:
+    - if: $CI_COMMIT_REF_NAME == "master"
     - if: $CI_COMMIT_REF_NAME =~ /^stg-v[0-9]+\.[0-9]+.*$/  # e.g. stg-v0.1
 
 .production-refs:                  &production-refs