Adds Dockerfile

This allows easy deployment in any docker capable OS (namely, things running a linux kernel that does lxc containers).

I have published this to the docker registry and it can be used by simply:

```
docker pull dscape/dillinger
```

Running it is as simple as

```
docker run -d dscape/dillinger
```

Curl it ;)

```
docker ps | grep dscape/dillinger | awk '{p}' | xargs -I '{}' docker port {} 8080 | xargs -I '{}' curl localhost:{}
```

Enjoy :)