Add deployed-app-image-repo plugin hook

This hook will allow users to modify the repo in use for deployed applications. An example use case would be one where the image being deployed is from a remote docker registry where the `dokku` username is taken and the app will be deployed from a separate username.

docs/development/plugin-triggers.md

@@ -177,6 +177,23 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 echo 'not-latest'
 ```
 
+### `deployed-app-image-repo`
+
+- Description: Used to manage the full repo of the image being deployed. Useful for deploying from an external registry where the repository name is not `dokku/$APP`
+- Invoked by: `internal function dokku_deploy_cmd() (deploy phase)`
+- Arguments: `$APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+APP="$1"
+# change the repo from dokku/APP to dokkupaas/APP
+echo "dokkupaas/$APP"
+```
+
 ### `deployed-app-repository`
 
 - Description: Used to manage the remote repository of the image being deployed.
@@ -211,7 +228,7 @@ cache-bust-build-arg() {
   local APP="$1" IMAGE_SOURCE_TYPE="$2"
   local output=""
 
-  if [[ "$IMAGE_SOURCE_TYPE" == "dockerfile" ]]; then 
+  if [[ "$IMAGE_SOURCE_TYPE" == "dockerfile" ]]; then
     output=" --build-arg CACHEBUST=$(date +%s)"
   fi
   echo -n "$STDIN$output"