only docker build if push to main

.harness/firebase-deploy.yaml

@@ -24,6 +24,7 @@ spec:
             env | grep ^build_
             git --no-pager diff --name-only $build_before..$build_after
       - name: dind
+        when: build.event == "push" && build.target == "main"
         type: background
         spec:
           container:
@@ -34,6 +35,7 @@ spec:
             path: /var/run
       - name: docker
         type: run
+        when: build.event == "push" && build.target == "main"
         spec:
           name: docker
           container: registry-app.inhome.blapointe.com:5001/docker:latest