Docker image out of date

The Docker image referred to in the documentation is very out of date and uses an old version of Hugann.

I am maintaining a docker container for myself here: https://registry.hub.docker.com/u/andrewcurioso/huginn/
At the very least I think the documentation should be updated to use my docker image and instructions since they are more current, more configurable (production ready), likely to remain current.

This might conflict with #416 so feel free to reject if it does not fit in the roadmap.

Steps to rebuild the image using the code in this Dockerfile:

```
git clone https://github.com/andrewcurioso/huginn.git
git checkout docker
docker build -t huginn .
```

From there you can follow the instructions on the Docker Hub page to run it.
It has the disadvantage of not being able to run without a MySQL container running (the other has MySQL built-in) but it is more consistent with Docker philosophy.