Currently, we are asking devs to set this env var manually before they run tests.
We may as well save them some keystrokes by setting it in the dev image
programmatically.
These changes make to possible to run:
tutor mounts add /path/to/my-xblock
The xblock directory with then be auto-magically bind-mounted in the
"openedx" image at build time, and the lms*/cms* containers at run time.
This makes it effectively possible to work as a developer on
edx-platform requirements.
We take the opportunity to move some openedx-specific code to a
dedicated module.
Close https://github.com/openedx/wg-developer-experience/issues/177
