feat: add ability to specify a custom Dockerfile for an app

plugins/builder-dockerfile/commands

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/builder-dockerfile/help-functions"
+
+case "$1" in
+  help | builder-dockerfile:help)
+    cmd-builder-dockerfile-help "$@"
+    ;;
+
+  *)
+    exit "$DOKKU_NOT_IMPLEMENTED_EXIT"
+    ;;
+
+esac