add gitlab ci file to build docker container

.gitlab-ci.yml

+stages:
+  - build-container
+
+variables:
+  KANIKO_BUILD_IMAGENAME: admin-backend
+
+build-container:
+  stage: build-container
+  image:
+    # We need a shell to provide the registry credentials, so we need to use the
+    # kaniko debug image (https://github.com/GoogleContainerTools/kaniko#debug-image)
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  script:
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
+    - /kaniko/executor --cache=true --context ${CI_PROJECT_DIR}/ --dockerfile ${CI_PROJECT_DIR}/Dockerfile --destination ${CI_REGISTRY_IMAGE}/${KANIKO_BUILD_IMAGENAME}:${CI_COMMIT_REF_NAME}
+