Revert "Temporarily disable scheduled docker build for faster testing"

This reverts commit 5afd280.

Author: podhrmic

.github/workflows/docker.yml

@@ -1,9 +1,8 @@
 name: docker
 
 on:
-  pull_request:
-  # schedule:
-    # - cron: '30 18 * * *'
+  schedule:
+    - cron: '30 18 * * *'
 
 env:
   CERBERUS_IMAGE_ID: ghcr.io/rems-project/cerberus/cn