GUACAMOLE-2037: Make listen address configurable

src/guacamole-docker.md

@@ -89,6 +89,23 @@ running outside of Docker, will be able to connect to guacd directly.
 
 (guacamole-docker-image)=
 
+### Enabling listening for IPv6 connections
+
+By default, guacd will bind to `0.0.0.0`, which means guacd will respond to
+IPv4 connections only.
+
+To allow guacd to also respond to IPv6 connections, the `LISTEN_ADDRESS`
+environment variable can be changed to `::`.
+
+Setting this variable to anything other than `0.0.0.0` (for IPv4-only) or
+`::` (for IPv4 and IPv6) is not supported. (And if you try, you'll probably
+break the health check, which is hardcoded to `127.0.0.1` and is not
+configurable.)
+
+This environment variable does not affect guacd's ability to connect to
+IPv6 servers, e.g. over RDP over SSH. It only affects the ability of
+guacamole-client to connect to guacd.
+
 The Guacamole Docker image
 --------------------------