workflows: move scheduled container rebuild into this repo

The container rebuild workflow has always lived here, but the scheduled
rebuild has historically run from the openslide/builds repo.  The sole
reason for this indirection is that openslide/builds has logic to keep the
repository active, so GitHub doesn't disable scheduled jobs after 60 days
of inactivity.  Now that we have periodic update-checking in this repo, we
have that problem here anyway:

    #144

Move the timer out of openslide/builds to simplify.

Signed-off-by: Benjamin Gilbert <[email protected]>

.github/workflows/container.yml

@@ -9,9 +9,9 @@ on:
   pull_request:
     branches: [main]
     paths: [builder/*]
+  schedule:
+    - cron: "40 14 1 * *"
   workflow_dispatch:
-  # openslide/builds invokes this from a scheduled build
-  workflow_call:
 
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