Add gitlab-ci.yml to automatically build docker images

.gitlab-ci.yml

+stages: 
+  - image
+docker-build:
+  stage: image
+  image: docker:latest
+  tags:
+    - privileged
+  only:
+    refs:
+      - master
+  services:
+    - docker:dind
+  variables:
+    DOCKER_TLS_CERTDIR: "/certs"
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  script:
+    - |
+      if [[ "$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}"