In case of production setup you need to build your custom image for installing apps. This repository only publishes frappe/erpnext image with no additional app. You CANNOT bench get-app in running containers.
To build image refer documentation to build custom apps. To automate it using CI refer this post.
In case of development setup, you can do bench get-app as usual.
How to build assets?
You cannot build assets using bench build in running production containers. It will mess up the attached assets volume for the container in which the command was executed. It will cause problems in serving static assets. The asset building process is done during the image build. Images come pre-packaged with built assets and there is no need to build assets in production environment.