Image build is optimized.

.gitlab-ci.yml

@@ -23,7 +23,8 @@ variables:
   DOCKER_CPUS: "2"
 
 before_script:
-  - echo $DOCKER_PW | docker login -u $DOCKER_USER --password-stdin
+  - mkdir -p ~/.docker
+  - echo $DOCKER_CONFIG_JSON | base64 -d > ~/.docker/config.json
   - docker system prune -a -f --volumes || true
   - docker container prune -f || true
   - docker network prune -f || true