Encountering OriginNotAllowed error during fresh install

Rookie Lemmy admin here, trying to get an instance off the ground. I've followed the docker install instructions located here: https://join-lemmy.org/docs/administration/install_docker.html

Everything starts correctly, no errors in any logs, I can access lemmy-ui at the correct domain as expected. But when I try to login nothing happens. If I check the lemmy logs I see the following error:

WARN Error encountered while processing the incoming HTTP request: lemmy_server::root_span_builder: Origin is not allowed to make this request

I'm pulling my container from dessalines/lemmy:latest, which means I'm running v0.18 (I'm installing on a fresh install of Ubuntu 22.04 LTS, if that matters). I see from the Lemmy change log there have been several changes related to CORS, including the addition of a LEMMY_CORS_ORIGIN environment variable. I thought maybe the example configs included in the installation documentation were out of date, so I added that environment variable to my docker compose file and restarted lemmy. No change in behaviour. I've tried setting that ENV to every combination of host/port I can think of with no success. I also enabled debug level logging on the lemmy docker, hoping that the log message would tell me the explicit origin that it was seeing so I'd know what to set the ENV to, but no luck.

Can anyone provide me with some tips? What am I supposed to set the LEMMY_CORS_ORIGIN variable to? Has anyone else encountered this error?