Create Docker container to host the xblock-sdk

This implements an initial Docker container to host an instance of the
OpenEdX XBlock SDK which is to be used during XBlock development.
> A phony target is one that is not really the name of a file; rather it
> is just a name for a recipe to be executed when you make an explicit
> request. There are two reasons to use a phony target: to avoid a
> conflict with a file of the same name, and to improve performance.

[1] https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html