Add .gitlab-ci.yml to build and push image

.gitlab-ci.yml

+docker-build-master:
+  # Official docker image.
+  image: docker:latest
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  script:
+    - docker build --pull -t "${CI_REGISTRY_IMAGE}:latest" .
+    - docker tag "${CI_REGISTRY_IMAGE}:latest" "${CI_REGISTRY_IMAGE}:archlinux"
+    - docker push "${CI_REGISTRY_IMAGE}:latest"
+    - docker push "${CI_REGISTRY_IMAGE}:archlinux"
+  only:
+    - master