rewrite Dockerfile:
* drop privileges
* write logs and runtime generated data to shared volume
* use custom entrypoint script to fix permissions of volumes
* specify HOME variable for node-gyp to emit fewer warning
* expose port 25
add ignore patterns for docker and git
provide minimalistic Haraka configuration to use the defined envirionment setup (volume paths and ports)
improve example to provide a minimalistic catch-all SMTP server
use GNU Make as build system