As it currently stands, anything in Wordpress that needs to send email is broken. The wp_mail function that is used for sending out email by default is a thin wrapper over PHP's mail function, which defaults to calling /usr/sbin/sendmail -t -i (cf. http://php.net/manual/en/mail.configuration.php). There are php.ini settings for SMTP, but they are only used on Windows.

For my own work based on this image, I've been installing ssmtp and configuring it to use an external relay host for SMTP, but that's clearly not a turnkey solution for everyone. It also looks like there are many Wordpress plugins that provide SMTP support, but I don't think that installing any of them by default in this image seems reasonable.

Perhaps all of this should just be documented, but I wanted to get it on your radar.