Run wp cron under "ionice"

docker/conf/services.d/wpcron/run

@@ -3,5 +3,5 @@
 # The "run-cron" execution takes a few minutes, so the actual period
 # comess out at something closer to 10 minutes.
 export BATCH_SIZE=300
-exec every 300 on-local-blogs run-cron >/dev/null
+exec nice ionice -c 3 every 300 on-local-blogs run-cron >/dev/null