The scheduled task is a pin.
I probed the CLI (the container and the host, which is Ubuntu 16.04 running Nginx which proxies to Discourse) and the admin interface for errors in the logs. All appears normal from the CLI in the Nginx and Discourse logs. The only recent errors in the Discourse Admin logs are related to the New Topic button theme component, which I have disabled whenever I am not trying to troubleshoot it.
I tried uploading the images again, as well as creating a new post; neither of which allow the images to be opened with a lightbox.