Avoid pushing postgres-operator image on PRs. (#52)

Do push as postgres-operator from the master branch, and as a
postgres-operator-test when evaluating pull-requests.

delivery.yaml

@@ -16,8 +16,15 @@ build_steps:
         export OPERATOR_TOP_DIR=$GOPATH/src/github.com/zalando-incubator
         mkdir -p $OPERATOR_TOP_DIR
         ln -s $(pwd) $OPERATOR_TOP_DIR/postgres-operator
-    - desc: 'Build & push'
+    - desc: 'Build & push docker image'
       cmd: |
         export PATH=$PATH:$HOME/go/bin
-        export IMAGE=registry-write.opensource.zalan.do/acid/postgres-operator
+        IS_PR_BUILD=${CDP_PULL_REQUEST_NUMBER+"true"}
+        if [[ ${CDP_TARGET_BRANCH} == "master" && ${IS_PR_BUILD} != "true" ]]
+        then
+          IMAGE=registry-write.opensource.zalan.do/acid/postgres-operator
+        else
+          IMAGE=registry-write.opensource.zalan.do/acid/postgres-operator-test
+        fi
+        export IMAGE
         make tools deps docker push