Add docker building ci

.gitlab-ci.yml

+image: docker:stable
+
+variables:
+  DOCKER_HOST: tcp://docker:2375/
+  DOCKER_DRIVER: overlay2
+  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
+
+services:
+  - docker:dind
+
+build:
+  stage: build
+  script:
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+    - docker pull $CI_REGISTRY_IMAGE:master
+    - docker build --cache-from $CI_REGISTRY_IMAGE:master -t $IMAGE_TAG .
+    - docker push $IMAGE_TAG
+