I just forgot something. Update the container while it is running.
$ docker update nextcloudpi $ADD-MISSING-OPTIONS
and the new options will be appended. Don't forgot to test everything is working by restarting.
$ docker restart nextcloudpi
Let's write a totally new run command.
Seems insane, but this is the correct way to use Docker.
$ docker stop nextcloudpi
Stops your running container; this takes Nextcloud offline manually.
$ docker rm nextcloudpi
Removes the container, also known as your run settings. This will have no impact on your /data directory (volume) or Nextcloud itself. This is purely changing things as Docker understands them. Obviously, you'll want to keep your volume location and other details the same as before.
$ history | grep docker
Can help you recall what you personally used as your run command. Or, we can use the default of Nextcloudpi:
$ docker run -d -p 4443:4443 -p 443:443 -p 80:80 -v ncdata:/data --name nextcloudpi ownyourbits/nextcloudpi-x86 DOMAIN
Your Nextcloud will now restart along with any changes you have made to your run command. Profit!
Enjoy, test any changes you make, and always keep a diary of your work in case you get confused.