Upgrade to the kaniko v0.20.0 image

The v0.20.0 image ships the fix for the missing platform metadata that
cause images generated with it to not work with `podman`:
https://github.com/GoogleContainerTools/kaniko/issues/997



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

.gitlab-ci.yml

@@ -49,13 +49,14 @@ build-base-rootfs:
 
 .build-docker-image:
   image:
-    name: gcr.io/kaniko-project/executor:debug-v0.16.0
+    name: gcr.io/kaniko-project/executor:debug-v0.20.0
     entrypoint: [""]
   only:
     - branches
   needs:
      - build-base-rootfs
   script: &build-docker-image-script
+    - mkdir /kaniko/.docker/
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
     - >
       /kaniko/executor