custom vendor update script location #1

i'm currently working on a third party Dockerfile where i need to define the vendor update script myself, because the original repo authors don't want to integrate Docker themselves up to now.

If i set the GIT_REPO variable as intended i have no possibility to append my custom update script between repository cloning and executing ./update.

So for now i'm forced to override the /bin/update script myself within my image to achieve the intended behaviour.

Adding a check for an additional environment variable (eg. UPDATE_SCRIPT_PATH) with a custom path for update should solve this.