More strangeness:
I rebuilt the Discourse container, and edited resolv.conf to be just nameserver 10.33.55.1, and I could immediately resolve hostnames correctly. But whenever I restart the container, the contents of resolv.conf changes to:
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 10.33.55.1
So something is running when the container starts, and adding the Google nameservers to resolv.conf. Or does the container filesystem revert to its previous state whenever it is restarted?