First of all, I would rather use docker-compose, not a single docker build, since your app can grow. Speaking of growing, I would separate dev & prod docker-compose.

> When we develop more than one python project, we need to configure our development environment with the dependencies of all the projects.

use pyenv and simple virtualenvs.

I think you are trying to substitute a virtualenvs with a docker. Why trying to kill a sparrow with a cannon?