To reduce the number of moving parts during the Buster rebase we forced
the use of the 19.03 image-builder Docker container image.

Let's now move back to using the image-builder matching the current
release (still named `next` until we switch to the final name which
should be `v2019dev0`).

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>