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.
So the final docker repository name is: `{d.RepoBaseName}/{basename(r.Name)}:{latest|revhash(HEAD)}`

I wonder why you can't just use `{d.RepoBaseName}` and dont tie docker repo name to source repository name.