Dockerize

Dockerized the testrunner so that buildpacks can run tests
within a container. A build pack projects would need to create
a Dockerfile similar to this:
```
FROM heroku/testrunner
ADD . /app/buildpack
```
And then run some commands like this:
```
docker build --rm -t heroku/buildpack-scala .
docker run -i heroku/buildpack-scala /app/testrunner/bin/run /app/buildpack
```
I'm not too sharp with docker so there might be a better way to do this. Plz advise! :)