Base element for creating images from docker containers.

This element is incomplete by itself, you’ll want to add additional elements, such as dpkg or yum to get richer features. At its heart, this element simply exports a root tarball from a named docker container so that other diskimage-builder elements can build on top of it.

The variables DISTRO_NAME and DIB_RELEASE will be used to decide which docker image to pull, and are required for most other elements. Additionally, the DIB_DOCKER_IMAGE environment variable can be set in addition to DISTRO_NAME and DIB_RELEASE if a different docker image is desired.