nginx/pages.conf

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    server_name ~^(\d)\.exozy\.me;

    include conf.d/ssl;

    index index.html;

    location / {
        # Must use 127.0.0.1 here since it's resolved at runtime
        # https://serverfault.com/questions/638505/nginx-dynamic-proxy-pass-doesnt-resolve-properly
        resolver 192.168.122.1;
        proxy_pass http://localhost:420$1;
    }
}

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    server_name ~^(?<user>.+)\.exozy\.me;

    include conf.d/ssl;

    root /srv/http/pages/$user;
    index index.html;

    error_page 404 /404.html;
    location = /404.html {
        internal;
    }

    location / {
        try_files $uri $uri.html $uri/ =404;
    }
}