Push built images to prod GCR

- We only push to prod, not staging, since chartpress isn't set up
  to do multiple pushes. jupyterhub/chartpress#25
  should help with that.
- Staging can read all prod images for now, to work around this.
- Create and use a new prod service acct that has image read / write perms.

.travis.yml

@@ -63,6 +63,9 @@ before_deploy:
   (cd mybinder && helm dep up)
 - |
   # Stage 3, Step 1: Build images if needed
+  # We build and push our misc images to prod only for now.
+  # Staging has readonly access to prod images.
+  docker login -u _json_key --password-stdin https://gcr.io < secrets/gcr-auth-key-prod.json
   chartpress --commit-range ${TRAVIS_COMMIT_RANGE}
   # Doing this here, since git crypt doesn't like unclean checkouts
 - |

chartpress.yaml

@@ -1,6 +1,6 @@
 charts:
   - name: mybinder
-    imagePrefix: jupyterhub/mybinder-extr
+    imagePrefix: jupyterhub/mybinder-tool-
     images:
       events-archiver:
         valuesPath: eventsArchiver.image