Add CI

.gitlab-ci.yml

+test_build_image:
+  image: docker:git
+  tags:
+  - docker
+  services:
+  - docker:dind
+  script:
+    - docker build .
+  except:
+    - tags
+
+build_image:
+  image: docker:git
+  tags:
+  - doppler-docker
+  services:
+  - docker:dind
+  script:
+    - docker login -u "gitlab-ci-token" -p "$CI_BUILD_TOKEN" $CI_REGISTRY
+    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_BUILD_TAG" .
+    - docker login -u "gitlab-ci-token" -p "$CI_BUILD_TOKEN" $CI_REGISTRY
+    - docker push "$CI_REGISTRY_IMAGE:$CI_BUILD_TAG"
+  only:
+    - tags