clarify how to use custom domain when rewriting images

Author: paigecalvert

docs/vendor/helm-native-v2-using.md

@@ -19,7 +19,7 @@ To support Helm chart installations with the KOTS `kots.io/v1beta2` HelmChart cu
 ## Task 1: Rewrite Image Names {#rewrite-image-names}
 
 Configure the KOTS HelmChart custom resource `values` key so that KOTS rewrites the names for both private and public images in your Helm values during deployment. This allows images to be accessed at one of the following locations, depending on where they were pushed:
-* The [Replicated proxy registry](private-images-about) (`proxy.replicated.com`)
+* The [Replicated proxy registry](private-images-about) (`proxy.replicated.com` or your custom domain)
 * A public image registry
 * Your customer's local registry
 * The built-in registry used in Replicated Embedded Cluster or Replicated kURL installations in air-gapped environments
@@ -39,11 +39,12 @@ You will use the following KOTS template functions to conditionally rewrite imag
 
 For any private images used by your application, configure the HelmChart custom resource so that image names are rewritten to either the Replicated proxy registry (for online installations) or to the local registry in the user's installation environment (for air gap installations or online installations where the user configured a local registry).
 
-To rewrite image names to the location of the image in the proxy registry, use the format `proxy.replicated.com/proxy/<app-slug>/<image>`, where:
+To rewrite image names to the location of the image in the proxy registry, use the format `<proxy-domain>/proxy/<app-slug>/<image>`, where:
+* `<proxy-domain>` is `proxy.replicated.com` or your custom domain. For more information about configuring a custom domain for the proxy registry, see [Using Custom Domains](/vendor/custom-domains-using).
 * `<app-slug>` is the unique application slug in the Vendor Portal
 * `<image>` is the path to the image in your registry
 
-For example, if the private image is `quay.io/my-org/nginx:v1.0.1`, then the image name should be rewritten to `proxy.replicated.com/proxy/my-app-slug/quay.io/my-org/nginx:v1.0.1`.
+For example, if the private image is `quay.io/my-org/nginx:v1.0.1` and no custom domain is used for the proxy registry, then the image name should be rewritten to `proxy.replicated.com/proxy/my-app-slug/quay.io/my-org/nginx:v1.0.1`.
 
 For more information, see the example below. 
 
@@ -63,10 +64,10 @@ spec:
   values:
     image:
     # If a registry is configured by the user or by Embedded Cluster/kURL, use that registry's hostname
-    # Else use proxy.replicated.com 
-      registry: '{{repl HasLocalRegistry | ternary LocalRegistryHost "proxy.replicated.com" }}'
+    # Else use proxy.replicated.com or your custom proxy registry domain
+      registry: '{{repl HasLocalRegistry | ternary LocalRegistryHost "my.registry.io" }}'
       # If a registry is configured by the user or by Embedded Cluster/kURL, use that registry namespace
-      # Else use the image's namespace at proxy.replicated.com
+      # Else use the image's namespace at the proxy registry domain
       repository: '{{repl HasLocalRegistry | ternary LocalRegistryNamespace "proxy/my-app/quay.io/my-org" }}/nginx'
       tag: v1.0.1
 ```
@@ -171,6 +172,7 @@ metadata:
 spec:
   values:
     image:
+      # Use proxy.replicated.com or your custom domain for the proxy registry 
       registry: '{{repl HasLocalRegistry | ternary LocalRegistryHost "proxy.replicated.com" }}'
       repository: '{{repl HasLocalRegistry | ternary LocalRegistryNamespace "proxy/my-app/ecr.us-east-1.amazonaws.com/my-org" }}/api'
       pullSecrets: