Add Gitlab CI config

.gitlab-ci.yml

+stages:
+  - image
+
+docker:
+  stage: image
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  cache:
+    key: "$CI_COMMIT_REF_SLUG"
+    paths:
+      - cargo/
+      - target/
+    policy: pull
+  script:
+    - mkdir -p /kaniko/.docker
+    - echo "{\"auths\":{\"$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_TAG
+  only:
+    - tags