Allow customised CKAN Docker images (fixes #1904)

The current Docker build for CKAN doesn't support the creation of customised images, because it doesn't allow for a custom `ckan.ini`, as explained in #1904. This PR fixes that, using the solution I described in the issue.

I include a second commit that deletes what looks like the remnants of a submodule that was not properly deleted some time ago. Deleting it completely was necessary to [build the image at the Docker Hub](https://registry.hub.docker.com/u/cygri/ckan/). The built image is available on the Docker Hub as `cygri/ckan:2.3a`.