Revisit dockerfile routine

[ctorgalson]

The j2 template for the dockerfile is now indescribably complex, but in spite of that is not easily modified for other use cases. Let's make the following changes:

1. Make the path to the template file configurable so that end-users can swap their own template in,
2. Make the default template file more opinionated so it lands quite close to one specific use case (e.g. proxy, redis, cron, postgres, letsencrypt, nextcloud, custom volume),

3. Provide a single variable to the template, something like this:

     nd_docker_compose_variables:
       cron: []
       db: []
       letsencrypt: []
       nginx: []
       redis: []
       volumes: []

   This will permit end-users to provide their own variables in case their template looks quite different, while still permitting a simple and usable default configuration.

[ctorgalson]

Closed by #36