Support for docker rootfs provider

This (backwards-compatible) change allows an API user to specify a docker image for a container's rootfs, by specifying the container's rootfs path as `docker:///user/repo-name#some-tag`. The previous functionality of specifying an empty rootfs or an absolute path to a directory is preserved (by checking for an empty URL scheme).

An additional flag is now required on start (`-overlays`), which controls where the non-Docker backend stores its data.