anchor.diff:  
   ### Using a Service Principal
   Here is an example that creates a Debian image with updated packages. Specify
  -all environment variables (`ARM_CLIENT_ID`, `ARM_CLIENT_SECRET`, 
  +all environment variables (`ARM_CLIENT_ID`, `ARM_CLIENT_SECRET`,
   `ARM_SUBSCRIPTION_ID`) to use a service principal.
   The identity you choose should have permission to create disks and images and also
   to update your VM.
  
positive.code: 
  <%= partial "partials/builder/azure/common/client/_Config-not-required.html" %>
  
  ### Azure chroot builder specific options 
  
  #### Required:
  <%= partial "partials/builder/azure/chroot/_Config-required.html" %>
  
negative.code:     "client_id": "{{user `client_id`}}",
      "client_secret": "{{user `client_secret`}}",
      "subscription_id": "{{user `subscription_id`}}",
  
      "image_resource_id": "/subscriptions/{{user `subscription_id`}}/resourceGroups/{{user `resource_group`}}/providers/Microsoft.Compute/images/MyDebianOSImage-{{timestamp}}",
  
      "source": "credativ:Debian:9:latest"
  