Image build is optimized.

.gitlab-ci.yml

@@ -23,7 +23,7 @@ variables:
   DOCKER_CPUS: "2"
 
 before_script:
-  - docker login -u $DOCKER_HUB_USERNAME -p $DOCKER_HUB_PASSWORD
+  - docker login -u $DOCKER_USER -p $DOCKER_PW
   - docker system prune -a -f --volumes || true
   - docker container prune -f || true
   - docker network prune -f || true