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.
There is an offical registry of images [1].

There probably are more minimal images we could use, including `FROM
scratch` (which is an emtpy tarball) if we wanted to compile statically
linked binaries without external dependencies :)

[1] https://registry.hub.docker.com/