doc/image-build.md: rephrase PKR_VAR_environment_root

because we don't show it anymore

Author: elelaysh

docs/image-build.md

@@ -150,9 +150,10 @@ In summary, Packer creates an OpenStack VM, runs Ansible on that, shuts it down,
 
 Many of the Packer variables defined in `openstack.pkr.hcl` control the definition of the build VM and how to SSH to it to run Ansible. These are generic OpenStack builder options
 and are not specific to the Slurm Appliance. Packer variables can be set in a file at any convenient path; the build example above
-shows the use of the environment variable `$PKR_VAR_environment_root` (which itself sets the Packer variable
-`environment_root`) to automatically select a variable file from the current environment, but for site-specific builds
-using a path in a "parent" environment is likely to be more appropriate (as builds should not be environment-specific to allow testing before deployment to a production environment).
+shows the use of a path in the **site** environment. This is the most appropriate as builds should be tested in **dev** or **staging** before deployment to a production environment.
+
+During stackhpc CI image builds, the environment variable `$PKR_VAR_environment_root` (which itself sets the Packer variable
+`environment_root`) is used to automatically select a variable file from the current environment; see `.github/workflows/fatimage.yml`.
 
 What is Slurm Appliance-specific are the details of how Ansible is run: