Add instructions for using Docker image for building APKs

[KeyWeeUsr]

This way (including the gradlew fix) I built a runnable app on Windows 8.1 (in the old Docker for Windows with VirtualBox backend).

[AndreMiras]

awesome work, thanks

[AndreMiras]

We can ignore the conditional build failing. Seems like a glitch:

('recipes modified:', set([u'python3']))
('recipes to build:', set([u'python3']))
incompatible with python3crystax
falling back to python2
('recipes to build (no broken):', set([u'python3']))
('requirements:', u'python3,python2')