CI: introduce weekly rebuilds of docker image

Docker image will be built & pushed every 7 days to ensure that
its packages are up-to-date.

.github/workflows/docker_image.yml

@@ -9,6 +9,8 @@ on:
     - 'scripts/properties.sh'
     - 'scripts/setup-android-sdk.sh'
     - 'scripts/setup-ubuntu.sh'
+  schedule:
+    - cron:  '0 2 * * 0'
 
 jobs:
   update: