Run workflow weekly to keep the image up to date

Author: TrafeX

.github/workflows/build.yaml

@@ -5,6 +5,8 @@ on:
     branches: [ master ]
     tags: ['*']
   pull_request:
+  schedule:
+    - cron: '0 2 * * 6'
 
 env:
   IMAGE_NAME: trafex/php-nginx