specify dind version

.gitlab-ci.yml

-x86:
+docker_builder:
   image: docker:latest
   stage: build
   services:
-    - docker:dind
+    - docker:20-dind
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
   # Default branch leaves tag empty (= latest tag)