Add `logrotate` file for `digdag`.

cookbooks/digdag/files/etc/logrotate.d/digdag

@@ -0,0 +1,13 @@
+/var/log/digdag.log
+{
+        rotate 4
+        weekly
+        missingok
+        notifempty
+        compress
+        delaycompress
+        sharedscripts
+        postrotate
+                /usr/lib/rsyslog/rsyslog-rotate
+        endscript
+}