Refactor Dockerfile with multi-stage build and fix the development environment

The development environment was not working properly since we upgraded to hawthorn because assets are built differently in production and in development.

This release fixed the develoment anvironment and comes with a complete rewrite of the Dockerfile to leverage multi-stage builds.

We identified a lot more improvements that could not be done because of design issues in Open edX. For example:

• get rid of the "node_modules" directory in the production image,
• avoid reinstalling everything after mounting sources as volumes in the development image.

We did not surrender on these improvements but will need to spend some time demonstrating what is wrong to edX and submitting Pull Requests' to fix the issues at their root.