Several different lines of inquiry have lead to requiring editing the settings.js file. I am not able to locate it. I am running Node-Red under Docker on Raspberry Pi. Search of internet indicates several others are stuck on this as well, and no definitive answer.
You can also edit settings.js while Node-RED is running as it's only read on startup. Restart after editing is enough. Of course in this case the bind mount needs to be configured and the docker instance restarted before doing the modifications.
Are you also using Portainer (highly recommended!)? If so, you can just go to the "Exec Console" under "Quick actions" next to your container. Then you get a terminal window within the container, and from there it will be at "/data/settings.js".
Portainer can also show you the volume mappings in case you no longer remember how it was set up (can be done with Docker commands too, but Portainer simplifies it greatly).