add flutter-web.gitlab-ci.yml

flutter/flutter-web.gitlab-ci.yml

+# Required variables:
+## APP_DIRECTORY: The directory of the app
+
+stages:
+  - build
+
+variables:
+  FLUTTER_WEB_IMAGE: registry.420joos.dev/flutter:3.19.0
+
+web_build:
+  stage: build
+  image:
+    name: ${FLUTTER_WEB_IMAGE}
+    pull_policy: always
+  tags:
+    - amd64
+  artifacts:
+    paths:
+      - web/
+    expire_in: 1 day
+  script:
+    - cd ${APP_DIRECTORY}
+    - flutter build web
+  after_script:
+    - mv app/build/web .