The persistent data is just the contents of config/ (configuration),
apps/ (installed apps), and data/ (user data). The source code should
not be made into a volume because it will cause persistence issues when
you try to update an image.

Currently the existing setup means that standard Docker-style updates
(where you start a new Docker image) won't actually update the source
code -- causing confusion and other issues.

This matches how ownCloud's setup works as well.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>