You could probably go for nextcloud.yourdomain.ddns.org, but I can’t guarantee that since I have no experience with ddns.org.

There is a lot of cheap registrars, I’ll let you check and select the one you want if you need a second domain.

Basically docker let you setup multiple (fake) computers inside your computer. For those that want to correct this, I know. But I’m trying to explain the top view here.

So, your computer (host) is hosting multiple containers, each containers will have there set of available ports. And as stated earlier each container will have an application that may or may not need to expose ports. But since each have their own ports you can have hundreds of containers using the port 80 INSIDE the container, an using 80100 to 80200 OUTSIDE the container (on your host). The only real limit would be the amount of ports available (somewhere along the lines of 65535).

There. Now go have fun and read up on docker and reverse proxy! Don’t forget to use ssl on your nextcloud instance since it’ll be facing the internet! And… Have fun! :-)