This removes the dependency on an externally generated docker image, and
should make it easier to update the docker image or make other changes
related to it.
* If the image doesn't exist, try pulling it from the main repo's
* Use debian:testing-slim as the base, might result in a slightly
* Prevent installation of packages which are only recommended, for an
even smaller image.
* Add recommendation to remove new image from source repository in
favour of the main repository's.
Acked-by: Alex Deucher <firstname.lastname@example.org> # v2