.gitlab-ci.yml: build the image with kaniko

see https://docs.gitlab.com/ee/ci/docker/using_kaniko.html

.gitlab-ci.yml

+image:
+  name: gcr.io/kaniko-project/executor:debug
+  entrypoint: [""]
+
+build:
+  stage: build
+  script:
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
+    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA --destination $CI_REGISTRY_IMAGE:latest