Generate docker image as part of CI pipeline

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.

v2:
* If the image doesn't exist, try pulling it from the main repo's
  registry.
* Use debian:testing-slim as the base, might result in a slightly
  smaller image.

v3:
* 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 <alexander.deucher@amd.com> # v2
12 jobs for ci-generate-docker-image in 15 minutes and 24 seconds (queued for 1 minute and 42 seconds)