The point of stable is lack of change. Why are you rebuilding at all?
The reason that it pulls the container is to keep it in parity with the requirements of the discourse code.
If you want to do something like enter the container without doing a pull you can
docker exec -it app bash
And if you wanted to add a plugin without rebuilding you could enter the plugin directory and clone it and then migrate the database, but you still need to make she your getting the right commit of the plugin for strangle) stable.