Drone-docker plugin

Basic drone-docker plugin work. Feedback appreciated.

Plugins in drone have the ability to modify the buildfile and add commands to be run
after the tests complete. This

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.
Can you add a test file similar to npm_test.go that asserts some simple facts about the docker commands you create?