Set a separate domain for OpenPanel UI

To set separate domain just for the OpenPanel UI, for example pejcic.rs:

1. Add the domain to Caddyfile

   DOMAIN="pejcic.rs" && cat << EOF >> "/etc/openpanel/caddy/Caddyfile"
   
   # START USERPANEL DOMAIN #
   $DOMAIN {
       reverse_proxy localhost:2083
   }
   
   http://$DOMAIN {
       reverse_proxy localhost:2083
   }
   # END USERPANEL DOMAIN #
   EOF
   

2. Create an empty file with the domain name so SSL can be generated and renewed:

   touch /etc/openpanel/caddy/domains/pejcic.rs.conf
   

3. Restart services

   docker restart caddy openpanel