circle.yml: deploy on a personal hub account

I would to do some changes in Scope and test my changes in Kubernetes,
following the guide on:
https://github.com/weaveworks/scope#using-weave-scope-with-kubernetes

For that, I need a Docker image of Scope hosted on a Docker hub and
reference that Docker image in my Kubernetes resource yaml file. Instead
of building Scope on my laptop and pushing it manually on my Docker Hub
account, I am using my CircleCI account to do this automatically.

This patch fixes the deployment target in circle.yml. It uses the
environment variable $DOCKER_USER from the CircleCI settings to guard
against unintended deployment. As before, only the "master" branch will
trigger the deployment.

This change should not impact the build of the official Scope Docker
image.

circle.yml

@@ -62,7 +62,10 @@ test:
 deployment:
   hub:
     branch: master
-    owner: weaveworks
     commands:
-      - docker login -e $DOCKER_EMAIL -u $DOCKER_USER -p $DOCKER_PASS
-      - docker push weaveworks/scope
+      - >
+        test -z "${DOCKER_USER}" || (
+          docker login -e $DOCKER_EMAIL -u $DOCKER_USER -p $DOCKER_PASS &&
+          (test "${DOCKER_USER}" == "weaveworks" || docker tag weaveworks/scope:latest $DOCKER_USER/scope:latest) &&
+          docker push weaveworks/scope
+        )