change to only build 3 times a month. caching stopped working so enable squashing to make the image smaller in one layer instead of multiple layers

.github/workflows/os.yml

@@ -7,7 +7,7 @@ name: os
 
 on:
   schedule:
-    - cron: '0 5 * * *'  # 5 am every day
+    - cron: '0 5 8,18,28 * *'  # 5 am three times every month
   push:
     paths:
         - 'Dockerfile'
@@ -96,6 +96,7 @@ jobs:
        #   labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha
           cache-to: type=gha,mode=max
+          squash: true
 
 
       # Sign the resulting Docker image digest except on PRs.