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)
Status Job ID Name Coverage
  Docker Image
passed #62289
debian-testing

00:09:00

 
  Build
passed #62292
xserver-1.13

00:00:44

passed #62293
xserver-1.14

00:00:37

passed #62294
xserver-1.15

00:00:27

passed #62295
xserver-1.16

00:00:27

passed #62297
xserver-1.17

00:00:27

passed #62299
xserver-1.18

00:00:27

passed #62300
xserver-1.18-clang

00:00:29

passed #62302
xserver-1.19

00:00:28

passed #62304
xserver-1.19-clang

00:00:31

passed #62290
xserver-1.20

00:00:51

passed #62291
xserver-1.20-clang

00:00:50