Add support for custom base images with FDO_BASE_IMAGE
By default we start with distro:
version as the base image onto which
the dependencies are installed. Where FDO_BASE_IMAGE is set we start
with that image instead. This allows for layering of base images, e.g.
gstreamer can generate an image that the gstreamer-plugins then use as
base image for their specific needs.
Fixes: #7 (closed)
Edited by Peter Hutterer