Use upstream debos container for the base rootfs

Using the docker container for the *current* apertis release to build
the base rootfs for all docker images leads to a bootstrapping issue. So
that should be avoided.

The options for that are:
* Use a docker image from a previous Apertis release
* Use a non-apertis image

Using a docker image from Apertis (e.g. previous stable) is workable but
just pushes the bootstrapping issue down the line. So not really that
helpful (at some point you'll reach a stable release that's not longer
supported and might be dropped).

So avoiding Apertis in the bootstrapping chain seems recommendable. For
that simply pick the upstream debos images so we can keep using debos;
That still requires a kvm based worker, but so would previous stable
apertis releases at this point.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

.gitlab-ci.yml

@@ -35,7 +35,11 @@ before_script:
 
 build-base-rootfs:
   stage: base rootfs
-  image: registry.gitlab.apertis.org/infrastructure/apertis-docker-images/$RELEASE-image-builder
+  image:
+    name: godebos/debos
+    entrypoint: [ "" ]
+  tags:
+    - kvm
   artifacts:
     paths:
       - '*-base/*.tar.gz'