create get_app_image_repo() to define our app image pattern and use it

plugins/apps/post-delete

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-APP="$1"
+source "$PLUGIN_PATH/common/functions"
+APP="$1"; IMAGE_REPO=$(get_app_image_repo $APP)
 
 [[ -n $APP ]] && rm -r "$DOKKU_ROOT/$APP" > /dev/null
 
@@ -8,4 +9,4 @@ APP="$1"
 # shellcheck disable=SC2046
 docker rm -f $(docker ps -a --no-trunc | egrep "dokku/${APP}:" | awk '{ print $1 }' | xargs) &>/dev/null || true
 # shellcheck disable=SC2046
-docker rmi $(docker images -q $IMAGE | xargs) &>/dev/null || true
+docker rmi $(docker images -q $IMAGE_REPO | xargs) &>/dev/null || true