Fix docker.yml syntax (#458)

Commit 3adb12e meant to set the Docker builds to run daily but the syntax was off by one colon. This commit fixes this.

.github/workflows/docker.yml

@@ -2,7 +2,7 @@ name: docker
 
 on:
   schedule:
-    - cron '30 18 * * *'
+    - cron: '30 18 * * *'
 
 env:
   CERBERUS_IMAGE_ID: ghcr.io/rems-project/cerberus/cn:release