feat: use entrypoint to automatically pip-install mounted edx-platform

[kdmccormick]

Implements "Option A" from openedx-unsupported/wg-developer-experience#152, specifically:

A. Entrypoint script:

• Write an entrypoint script (we'll call it set-up-and-run). It should first do the ensure-setup thing. Then, it should run the arguments passed to it as a shell command.
  • For example: set-up-and-run ./manage.py lms runserver ... would create the egg-info if necessary, and then pass control to ./manage.py ....
• Add this script to the /openedx/bin folder of the openedx-dev image.
• Set the openedx-dev image's ENTRYPOINT to ['set-up-and-run'].
• Now, check that a mounted edx-platform can be run without needing to manually create the egg-info.
• Update the documentation to remove pip install .... from the mounted edx-platform setup steps.
  ...

You can compare this with Option B here: #25

[kdmccormick]

Closed in favor of #25