Add Docker-based dev environment instructions

[aronasorman]

Since we now have some developers regularly using a docker-based dev workflow, it'll be valuable to add that in as a dev setup option.

[Aju100]

Hi @aronasorman is it still open, I am new at learningequality and wanted to be part of and contributed to the project. If it's open, can you please give resources to get started and work on this issue.