gcloud docker -a do set .dockercfg, not .docker/config.json

What steps will reproduce the problem?

use gcloud docker on a node which has docker 1.7 setup with some dockerhub 
credentials allready registered
try to push to google docker registry

What is the expected output? What do you see instead?
Fail to push : 503
gcloud do setup credentials in .dockercfg, not .docker/config.json that is used 
by docker 1.7+ 
nb: docker do support .dockercfg for backward compat, but as long as 
.docker/config.json has been created this file will be preferred.

What is the output of 'gcloud info'?

Google Cloud SDK [0.9.67]

Platform: [Mac OS X, x86_64]
Python Version: [2.7.10 (default, Jun 10 2015, 19:42:47)  [GCC 4.2.1 Compatible 
Apple LLVM 6.1.0 (clang-602.0.53)]]
Site Packages: [Disabled]

Installation Root: [/Users/nicolas/Boulot/cloudbees/google/google-cloud-sdk]
Installed Components:
  core: [2015.06.30]
  core-nix: [2015.06.02]
  kubectl: []
  gcutil-msg: [2015.06.09]
  gcloud: [2015.06.30]
  gsutil-nix: [4.12]
  gsutil: [4.13]
  bq: [2.0.18]
  bq-nix: [2.0.18]
  kubectl-darwin-x86_64: [0.18.1]

Please provide any additional information below.

Original issue reported on code.google.com by nicolas....@gmail.com on 1 Jul 2015 at 4:14

prototipando / google-cloud-sdk

gcloud docker -a do set .dockercfg, not .docker/config.json #155