docs: clarify port 1010 bind on Docker for shellypro3em_old (#368)

* docs: clarify port 1010 bind on Docker for shellypro3em_old

The Docker image runs as a non-root user, so binding to port 1010
fails even with network_mode: host. Note the two working workarounds
(lower net.ipv4.ip_unprivileged_port_start or run as root) and call
out that bridge port-publishing does not forward UDP broadcasts.

* docs: note that combined shellypro3em is also affected by 1010 bind

The combined shellypro3em device type expands to both _old and _new
listeners, so it binds 1010 as well.

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: tomquist

README.md

@@ -1012,6 +1012,8 @@ A: Ports below 1024 require root privileges on Linux. Solutions:
 - Use `setcap` to grant permissions
 - Run as root (not recommended)
 
+Note: the Docker image runs as a non-root user, so binding port 1010 (used by `shellypro3em_old` and the combined `shellypro3em`, which starts both listeners) still fails with `PermissionError: [Errno 13]` under `network_mode: host`. Port 2220 (`shellypro3em_new`) is unaffected. Either lower the host's privileged-port range (`sudo sysctl -w net.ipv4.ip_unprivileged_port_start=1010`, persist via `/etc/sysctl.d/`) or run the container as root (`user: "0:0"` in compose). Publishing the port via bridge networking does **not** work, because the Marstek discovery packets are UDP broadcasts to the subnet address and aren't forwarded by Docker's port mapping.
+
 ### I get parsing errors on startup or the app crashes.
 
 A: Common causes: