Use default docker builder

.gitlab-ci.yml

+# This file is a template, and might need editing before it works on your project.
+# To contribute improvements to CI/CD templates, please follow the Development guide at:
+# https://docs.gitlab.com/ee/development/cicd/templates.html
+# This specific template is located at:
+# https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Docker.gitlab-ci.yml
+
+# Build a Docker image with CI/CD and push to the GitLab registry.
+# Docker-in-Docker documentation: https://docs.gitlab.com/ee/ci/docker/using_docker_build.html
+#
+# This template uses one generic job with conditional builds
+# for the default branch and all other (MR) branches.
+
+docker-build:
+  # Use the official docker image.
+  image: docker:latest
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  # Default branch leaves tag empty (= latest tag)
+  # All other branches are tagged with the escaped branch name (commit ref slug)
+  script:
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag=""
+        echo "Running on default branch '$CI_DEFAULT_BRANCH': tag = 'latest'"
+      else
+        tag=":$CI_COMMIT_REF_SLUG"
+        echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
+      fi
+    - docker build --pull -t "$CI_REGISTRY_IMAGE${tag}" .
+    - docker push "$CI_REGISTRY_IMAGE${tag}"
+  # Run this job in a branch where a Dockerfile exists
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile