docker: Make it easy to disable the GUI, document it (#6021)

README-Docker.md

@@ -18,7 +18,11 @@ $ docker run -p 8384:8384 -p 22000:22000 \
     syncthing/syncthing:latest
 ```
 
-Note that local device discovery will not work with the above command, resulting in poor local transfer rates if local device addresses are not manually configured.
+## Discovery
+
+Note that local device discovery will not work with the above command,
+resulting in poor local transfer rates if local device addresses are not
+manually configured.
 
 To allow local discovery, the docker host network can be used instead:
 
@@ -32,3 +36,24 @@ $ docker run --network=host \
 Be aware that syncthing alone is now in control of what interfaces and ports it
 listens on. You can edit the syncthing configuration to change the defaults if
 there are conflicts.
+
+## GUI Security
+
+By default Syncthing inside the Docker image listens on 0.0.0.0:8384 to
+allow GUI connections via the Docker proxy. This is set by the
+`STGUIADDRESS` environment variable in the Dockerfile, as it differs from
+what Syncthing would otherwise use by default. This means you should set up
+authentication in the GUI, like for any other externally reachable Syncthing
+instance. If you do not require the GUI, or you use host networking, you can
+unset the `STGUIADDRESS` variable to have Syncthing fall back to listening
+on 127.0.0.1:
+
+```
+$ docker pull syncthing/syncthing
+$ docker run -e STGUIADDRESS= \
+    -v /wherever/st-sync:/var/syncthing \
+    syncthing/syncthing:latest
+```
+
+With the environment variable unset Syncthing will follow what is set in the
+configuration file / GUI settings dialog.