gitlab-ci: Fix job dependencies

The `.build-docker-image` template had an explicit `needs:` pointing to
`build-base-rootfs` which was only correct for `build-base-docker-image`
as other build jobs actually depend on `build-base-docker-image`.

Most of the build jobs did indeed overrid `needs:` except
`build-image-builder-docker-image` and
`build-flatdeb-builder-docker-image` which ended up having a race
condition which caused them to fail if they got executed after
`build-base-rootfs` was done but before `build-base-docker-image` pushed
the base image.

Fix that by getting rid of the `needs:` and simply relying on stages
for sequencing the build jobs.

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

.gitlab-ci.yml

@@ -57,8 +57,6 @@ build-base-rootfs:
     entrypoint: [""]
   only:
     - branches
-  needs:
-     - build-base-rootfs
   script:
     - mkdir /kaniko/.docker/
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
@@ -101,16 +99,12 @@ build-base-docker-image:
 build-package-builder-docker-image:
   extends: .build-docker-image
   stage: derived images
-  needs:
-     - build-base-docker-image
   variables:
     image: package-builder
 
 build-package-source-builder-docker-image:
   extends: .build-docker-image
   stage: derived images
-  needs:
-     - build-base-docker-image
   variables:
     image: package-source-builder