This simplifies the tedious task of building all the Rockbox
toolchains manually by providing a build code for a Docker container
image. It's useful for quickly spinning up a build client with just a
couple commands and no waiting to compile (though downloading takes a
little while).
I've built an image as built1n/rbclient on Docker Hub.
All toolchains (even the weird ones) are included, except android16.
Change-Id: I6b863628ffb397604f59ec6def2f8bb8c8c7185f
