Parameterise the source folder location

This allows the user to perform property expansion via the maven-resources-plugin if they so desire. Also some user might prefer src/main/resources/docker.

This kind of removes the need to perform property expansion in this plugin.

It also questions the need for the prepare() phase and the packaging section of the conf.yml as the user can decide independently what artefacts to copy to the configuration folder. A matter of balancing usability with KISS/DRY?