Some our our respositories have Dockerfiles with commands like ADD . /path. These don't
work if we do - < Dockerfile because the only thing in "." will be the Dockerfile, not
the other things we want. This allows us to support that use case.
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.
