use same image for all builds

.gitlab-ci.yml

+image: docker003.vxs:5000/vsc/singularity-builder:latest
+
 before_script:
   - pwd
   - docker info
   - mkdir -p ~/.docker && echo "$DOCKER_AUTH_CONFIG" > ~/.docker/config.json
 
 build-minimal:
-  image: docker003.vxs:5000/vsc/singularity-builder:latest
   # only:
     # changes:      # changes does not work with variables atm!
     #   - minimal/**/*