Revert latest change: No CI container proxy usage for now.

.gitlab-ci.yml

-image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/node:12.16.3
-
-before_script:
-  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+image: node:12.16.3
 
 variables:
   MONGODB_VERSION: '4.2.6'
@@ -23,7 +20,7 @@ test_client:
 test_server:
   # Info: http://docs.gitlab.com/ee/ci/docker/using_docker_images.html#what-is-a-service
   services:
-    - name: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/mongo:$MONGODB_VERSION
+    - name: mongo:$MONGODB_VERSION
       alias: mongodb
   script:
     - cd app/server