Just to add to Dawid’s answer…
Docker can’t use “paths” to work out which repository to use, so you can’t have https://nexus.example.com/docker/repo1 and https://nexus.example.com/docker/repo2 or whatever. Instead, you have to use port numbers, so you’d open (say) port 9000 for repo1 and 9001 for repo2. Each repository has to be created with a port number, and they have to be unique. That’ll give you http://nexus.example.com:9000 and http://nexus.example.com:9001 (although docker command lines omit the https:// prefix).
In my case, I’m running an nginx reverse proxy in front of the Nexus web port (so I can use TLS, a certificate, etc). Since I’m doing that, I also use nginx to “reverse proxy” the docker ports too. So in my nginx config I’ve got some stuff about port 443, port 9000, 9001 etc. If I remember correctly, you have to use TLS with Docker, so you’ll need to make sure you provide TLS somewhere/somehow in your setup.