Document LISTEN_IP environment variable from plausible/analytics#1190.

Also explain how to fully lock down the services that Plausible requires
(`beam` inter-node communication and `epmd`).

docs/self-hosting-configuration.md

@@ -29,6 +29,7 @@ Following are the variables that can be used to configure the availability of th
 |----------------------|-------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | BASE_URL             | --          | The hosting url of the server, used for URL generation. In production systems, this should be your ingress host. |
 | PORT                 | 8000        | The port on which the server is available.                                                                                                                                                          |
+| LISTEN_IP            | 0.0.0.0     | The IP address on which the server is listening. `0.0.0.0` means all interfaces, `127.0.0.1` means localhost. When changing this, you may also wish to change the Erlang VM's and Port Mapper Daemon's listen IP, using environment variables [`RELEASE_VM_ARGS`](https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-environment-variables) being e.g. `-kernel inet_dist_use_interface "{127,0,0,1}"` and [`ERL_EPMD_ADDRESS`](https://erlang.org/doc/man/epmd.html#environment-variables) being e.g. `127.0.0.1`. |
 | SECRET_KEY_BASE      | --          | An internal secret key used by [Phoenix Framework](https://www.phoenixframework.org/). Follow the [instructions](https://hexdocs.pm/phoenix/Mix.Tasks.Phx.Gen.Secret.html#content) to generate one. |
 | DISABLE_AUTH         | false       | Disables authentication completely, no registration, login will be shown.                                                                                                                           |
 | DISABLE_REGISTRATION | false       | Disables registration of new users, keep your admin credentials handy ;)                                                                                                                            |