Add wp-cron to the container

docker/conf/services.d/wpcron/run

+#!/bin/sh
+# Run WP-Cron periodically.
+# 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 2>&1
+