This makes the container file element search the active element list
for `containerfiles/${DIB_RELEASE}` for building. This makes it easy
to write wrappers for ubuntu/fedora/etc. containerfile elements.
Change-Id: I68f1d928e54a70bad76985ddd3e156bb5f978b0d
This is a base element which uses a containerfile (Dockerfile) to
build a container image, then the filesystem is extracted from that
image and forms the root of the dib image.
You can add as little or as much to the dockerfile as desired.
Change-Id: I4e821aa2ce7feb8841ef31da56de1a31aa9218b5