docker: add cron service to compose file.

config/initializers/clockwork.rb

@@ -0,0 +1,22 @@
+# Define cronjobs using the clockwork gem; see https://github.com/Rykian/clockwork.
+# Use `bin/rails danbooru:cron` to start the cron process.
+#
+# See also `app/logical/danbooru_maintenance.rb`.
+
+module Clockwork
+  every(1.hour, "hourly", at: "**:00") do
+    DanbooruMaintenance.hourly
+  end
+
+  every(1.day, "daily", at: "00:00") do
+    DanbooruMaintenance.daily
+  end
+
+  every(1.week, "weekly", at: "Sunday 00:00") do
+    DanbooruMaintenance.weekly
+  end
+
+  every(1.month, "monthly", at: "00:00") do
+    DanbooruMaintenance.monthly
+  end
+end