OCPBUGS13238: CoreOS Layering docs should have info about using the cluster pull-secret

Author: Michael Burke

modules/coreos-layering-configuring.adoc

@@ -44,6 +44,8 @@ RUN rpm-ostree override replace https://example.com/hotfixes/haproxy-1.0.16-5.el
 Instructions on how to create a Containerfile are beyond the scope of this documentation.
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+* Because the process for building a custom layered image is performed outside of the cluster, you must use the `--authfile /path/to/pull-secret` option with Podman or Buildah. Alternatively, to have the pull secret read by these tools automatically, you can add it to one of the default file locations: `~/.docker/config.json`, `$XDG_RUNTIME_DIR/containers/auth.json`, `~/.docker/config.json`, or `~/.dockercfg`. Refer to the `containers-auth.json` man page for more information. 
+
 * You must push the custom layered image to a repository that your cluster can access.
 
 .Procedure