Docker and the OCI container ecosystem

The templates currently offer two modes of operation for generating images: A simple one where one can just specify the base system packages to be installed, and a generic one which allows running a script to set up the filesystem arbitrarily.

The templates produce a single layer, but it can be based on any other image (by default a standard image of one of the supported base distros), which is included as separate layers. This allows efficient sharing of common base contents.