docker: Replace Apache with gunicorn as app server

CKAN's install instructions have, up until now, recommended the use of
both Nginx and Apache as part of the default install. While both are
venerable pieces of software, using both to deploy a single application
is unusual to say the least.

Nginx is used as a battle-hardened frontline webserver and cache, while
Apache is used as a WSGI application server.

Better WSGI application servers exist, and one of them is gunicorn.
Gunicorn is a smaller application, focused on only one thing (WSGI
serving) and has much more predictable performance characteristics than
Apache.