Add docker.io Drone plugin which creates a docker image for each successful build

This change adds a new Drone plugin for creating a docker image. The plugin
creates a docker image for the test build and pushes the image to images.docker.io.

I've just tested it end-to-end with 'drone build' from the command line because
I'm not sure of another test that would be useful.
Add tests for docker plugin

These tests just check that the docker commands show up in the buildfile