Building using OverlayFS and systemd-nspawn

[stuartpb]

Also affects release and local deployment.

So, per the discussion in https://github.com/plushu/plushu-apps/issues/12#issuecomment-69482581, it would be a massive improvement if the app lifecycle could be reworked to just use a couple ephemeral layers without tracking or versioning (with slug creation on demand).

It seems like OverlayFS would be a good fit for this: http://askubuntu.com/questions/109413/how-do-i-use-overlayfs (and following from http://www.phoronix.com/scan.php?page=news_item&px=CoreOS-Btrfs-To-EXT4-OverlayFS)

Things that would be better with this:

• Images are a lot less fussy to manage: they just naturally exist as 3 layers of dir.
• Linking (for addons) could use a less ridiculous mechanism then eight-word-long env vars.

Things that would need to be reworked:

• A bunch of assumptions about the app being a Docker container (possibly fixable by hookifying, eg. a hook to get the app's IP instead of calling docker directly).
• Starting from the Cedarish image.
• Networking, possibly.

My thoughts:

• overlay-dirs could be a Plushu plugin that supports using aufs, btrfs's overlay stuff, or OverlayFS to combine dirs onto a dir (either based on availability or as separate plugins). I'd use another abstraction (like something from Docker's underlying lib), but I don't think this specific tool exists. (Worth checking for.)
• There'd be a "layers" directory under the app dir to keep these layers.

[stuartpb]

see also plushu/plushu-buildstep#1 (another possible approach, using Docker and overlayfs for a volume)