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 create an automated (trusted) build in Docker Hub, possibly building off release tags instead of git master, thereby providing users of this simpler deployment method, which is essentially an 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.