Docker image out of date

The Docker image referred to in the documentation is very out of date and uses an old version of Hugann.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.

## Notes

1. It has the disadvantage/advantage 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.
2. The previous Docker container checked out the source tree inside of the image itself. This one opts to be run from within a checked out version so that it can easily  be used for testing and development.