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.
Shouldn't this be the same image id, just a different tag? What if the Docker server is configured to clear the build-cache after each successful build (can't see why, but it's deffinitely possible)

```
f.WriteCmd(fmt.Sprintf("docker -H %s build -t %s %s", dockerServerUrl, dockerRepo, dockerPath))
f.WriteCmd(fmt.Sprintf("docker -H %s tag %s:latest %s:$(git rev-parse --short HEAD)", dockerRepo, dockerRepo))
```