The join script is not run inside the container. It is run on the outside (because not all containers can run join scripts - not all containers are based on appbox).
You want a script that is run inside the container. In this case,
setup would be the correct one.
I used this opportunity to add a snippet to our documentation about the various scripts:
Note that for appbox Apps, you could just download the packages on a full UCS server with unmaintained enabled (
ucr set repository/online/unmaintained=yes && apt update && apt download ...) and then upload these packages to your repository in the App Center.