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.