1957 improved dockerfiles

Hi all,

As discussed in issue 1957, I've made a few improvement to the Docker files & documentation.
I've also created a development docker file (working with a shared volume), Fig service definitions, and a Vagrant config.

Changelog
* All containers extend the same base image phusion/baseimage (updated to 0.9.15), which means you only pull & cache them once.
* The Postgres container installs PostGIS, configures the database, the datastore & PostGis on the clan database.
* The Solr container has been updated to 4.10.1
* The CKAN container has been updated to configure the datapusher
* An example of custom container enabling other extensions in the contrib folder. The example it relies on a the redis container.
* A lightweight development container working with a shared volume so you can make live changes
* A default Fig service definition that builds the docker containers using the default package
* A development Fig service definition that builds the docker containers
* A development Vagrant config file
* Updated doc

Possible other improvements:
* I think Nginx should be removed from the main container to use the Official build of Nginx https://registry.hub.docker.com/_/nginx/
* I've not had a change to experiment with ONBUILD instructions, but that could be a better way of adding extensions when creating custom dockerfiles

Issue:
https://github.com/ckan/ckan/issues/1957