Adjust Dockerfile to new version of pipenv

[reihl]

pipenv has been change and now you need to use pipenv requirements to create requirements.txt file

[sleviim]

@nirs Hi Nir, Can we merge this one? Otherwise, students will need to run it manually

[nirs]

Getting rid of pipenv is better, but if this works it is an easier change. This project does not need pipenv locking. pip install with requirements.txt file is better and much faster.

[yaacov]

Hi guys, we have a container friendly version in, no pipenv is used: https://github.com/RedHat-Israel/rose-game-engine