Added Dockerfile & Fig definition

Hi,

I've added a  Dockerfile & Fig definition to run the datapusher in a Docker container.
This is going to be needed for the upcoming ckan-docker config.

I've already tested it and I've got it built on the docker hub until it's available under the CKAN organisation.
https://registry.hub.docker.com/u/clementmouchet/datapusher/

Cheers,

Clement