{
  "variables": {
    "project_id": "{{env `PROJECT_ID`}}"
  },
  "builders" : [
    {
    "type": "googlecompute",
    "project_id": "{{user `project_id`}}",
    "source_image": "backports-debian-7-wheezy-v20150603",
    "zone": "us-central1-f",
    "image_name" : "guestbook-image",
    "disk_size" : 10,
    "machine_type" : "n1-standard-2",
    "network" : "jenkins-net",
    "tags" : ["ssh"]
    }
  ],
  "provisioners" : [
    {
    "type": "shell",
    "inline": [
      "apt-get -y -qq update",
      "wget -q -O docker-script.sh https://get.docker.com/ ",
      "chmod +x docker-script.sh",
      "./docker-script.sh",
      "rm docker-script.sh",
      "gcloud -q components update",
      "gcloud docker pull gcr.io/{{user `project_id`}}/guestbook"
      ]
    }
  ]
}

{
  "variables": {
    "project_id": "{{env `PROJECT_ID`}}",
    "gcr_prefix": "{{env `GCR_PREFIX`}}"
  },
  "builders" : [
    {
    "type": "googlecompute",
    "project_id": "{{user `project_id`}}",
    "source_image": "backports-debian-7-wheezy-v20150603",
    "zone": "us-central1-f",
    "image_name" : "guestbook-image",
    "disk_size" : 10,
    "machine_type" : "n1-standard-2",
    "network" : "jenkins-net",
    "tags" : ["ssh"]
    }
  ],
  "provisioners" : [
    {
    "type": "shell",
    "inline": [
      "apt-get -y -qq update",
      "wget -q -O docker-script.sh https://get.docker.com/ ",
      "chmod +x docker-script.sh",
      "./docker-script.sh",
      "rm docker-script.sh",
      "gcloud -q components update",
      "gcloud docker pull {{user `gcr_prefix`}}/{{user `project_id`}}/guestbook"
      ]
    }
  ]
}

Add support for GCR_PREFIX environment variable
Add support for GCR_PREFIX environment variable