part of the problem might be that the documentation for Docker seems to be outdated. Some time ago, Rasa Core and Rasa NLU were merged and the wording inside might lead you in a wrong direction.
As a workaround, I suggest you use an official Ubuntu image and derive your own environment from it. Since you are emulating a fully packged Unix system, Rasa installation and configuration could easily be done via the Dockerfile.
A good starting point might be the official Rasa Dockerfile but keep in mind, that in my suggestion, there is no need to use rasa as an entrypoint.
If you want to follow the documentation however, you might need to use the official Dockefile and figure out, how to use it. Most of the commands seem to be accurate.