Setup GitLab CI Docker image build.

.gitlab-ci.yml

+# This file is a template, and might need editing before it works on your project.
+# Official docker image.
+image: docker:latest
+
+services:
+  - docker:dind
+
+.build-main:
+  stage: build
+  script:
+    # - cd $TAG
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+    - docker build --pull -t "$CI_REGISTRY_IMAGE:$TAG" .
+    - docker push "$CI_REGISTRY_IMAGE:$TAG"
+  only:
+    - main
+  tags:
+    - docker-build
+
+build:
+  extends: .build-main
+  variables:
+    TAG: "latest"
+
+build-branch:
+  stage: build
+  script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" .
+    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
+  except:
+    - main
+  tags:
+    - docker-build