Add a way for automically installing custom apps

[simwai]

Issue: Feature Request
I'm always frustrated when I need to build a custom Docker image, just to add a custom add-on. It would be so much easier to only make it necessary to create an apps.json file and reference it by an env var called FRAPPE_APPS and then the add-ons get automatically installed on init of the docker compose yml file. Thanks for checking out this feature request.