docs: README reverse-proxy section matches the NPM network reality

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -61,9 +61,18 @@ Ports (bound to `127.0.0.1` on the host): nginx `8085` → `:80`, db `3310` →
 ### Reverse proxy (Nginx Proxy Manager)
 
 Add a proxy host: `dmtools.fvandenberg.nl` → forward to host `dmtools-nginx`,
-port `80`, scheme `http`; request a Let's Encrypt certificate. NPM and
+port `80`, scheme `http`; request a Let's Encrypt certificate.
-`dmtools-nginx` must share the external `docker_default` Docker network (the
+
-compose file attaches nginx to it as `proxy`).
+NPM must be able to resolve `dmtools-nginx`, i.e. be on the `dmtools_default`
+network. `setup.sh` connects it (`nginx_app_1` by default; override with
+`NPM_CONTAINER=...`); to do it by hand:
+
+```bash
+docker network connect dmtools_default nginx_app_1
+```
+
+NPM keeps that connection across restarts; it's only needed again after a
+`docker compose down` drops the network.
 
 ### Common commands