Try not to use any specific container registry

.gitlab-ci.yml

-before_script:
-  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
-  - docker pull mongodb:$MONGODB_VERSION
-  - docker tag mongodb ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/mongodb:$MONGODB_VERSION
-  - docker push mongodb
-
 image: node:12.16.3
 
 # Info: http://docs.gitlab.com/ee/ci/docker/using_docker_images.html#what-is-a-service