fix ruby version in CI

.gitlab-ci.yml

@@ -1,17 +1,17 @@
 before_script:
-    - apt-get update -qy
-    - apt-get install -y ruby git
-    - gem install dpl
-    - curl https://cli-assets.heroku.com/install.sh | sh
+  - apt-get update -qy
+  - apt-get install -y git
+  - gem install dpl
+  - curl https://cli-assets.heroku.com/install.sh | sh
 
 stages:
-    - deploy
+  - deploy
 
 deploy_production:
-    stage: deploy
-    image: ruby:latest
-    script:
-        - dpl --provider=heroku --app=$HEROKU_APP_PRODUCTION --api-key=$HEROKU_API_KEY
-        - heroku run bundle exec rails db:migrate --exit-code --app $HEROKU_APP_PRODUCTION
-    only:
-        - master
+  stage: deploy
+  image: ruby:3.0.1
+  script:
+    - dpl --provider=heroku --app=$HEROKU_APP_PRODUCTION --api-key=$HEROKU_API_KEY
+    - heroku run bundle exec rails db:migrate --exit-code --app $HEROKU_APP_PRODUCTION
+  only:
+    - master