Run gollum via docker

Docker is increasingly becoming a popular tool of choice among the devops community. Running gollum is now much simpler using docker. This PR contains the Dockerfile to build a gollum docker image. I suggest the maintainers to created a automated (trusted) build in Docker Hub, possibly building off release tags instead of git master, thereby providing users of this deployment method, which is simply a one-line command on any machine with docker installed:

```
docker run -p 4567:4567 -v ~/mywiki:/data gollum
```

I can squash the commits if this PR will be merged.