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

Dockerfile

@@ -24,4 +24,5 @@ ENV PUID=1000 PGID=1000
 HEALTHCHECK --interval=1m --timeout=10s \
   CMD nc -z localhost 8384 || exit 1
 
-ENTRYPOINT ["/bin/entrypoint.sh", "-home", "/var/syncthing/config", "-gui-address", "0.0.0.0:8384"]
+ENV STGUIADDRESS=0.0.0.0:8384
+ENTRYPOINT ["/bin/entrypoint.sh", "-home", "/var/syncthing/config"]

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.

README.md

@@ -62,6 +62,10 @@ There are a few examples for keeping Syncthing running in the background
 on your system in [the etc directory][3]. There are also several [GUI
 implementations][11] for Windows, Mac and Linux.
 
+## Docker
+
+To run Syncthing in Docker, see [the Docker README](16).
+
 ## Vote on features/bugs
 
 We'd like to encourage you to [vote][12] on issues that matter to you.
@@ -110,4 +114,5 @@ All code is licensed under the [MPLv2 License][7].
 [13]: https://github.com/syncthing/syncthing/blob/master/GOALS.md
 [14]: assets/logo-text-128.png
 [15]: https://syncthing.net/
+[16]: https://github.com/syncthing/syncthing/blob/master/README-Docker.md