TODO

  New capabilities for synth:

Use bcf (https://github.com/luislavaire/bcf)
    No more configuration file parsing.

synthfile
    This will be the main configuration file, here you can
    configure what kind of image you want, which distribution,
    which packages and so on.

.cache/
    If this directory exists, it is used as package cache
    directory for the builds. This directory is bind
    mounted into the image at build time to speed up
    building images.

overlay/
    If this directory exists, then synth will copy everything
    inside it into the images built. You can place arbitrary
    directory hierarchies in here. This is the best way to drop
    static files into the image, or override default ones.

post (executable)
    If this executable script exists, it is invoked inside the
    image (inside a runch invocation) and can adjust the image
    at a very late point.