Add note to manhole.md about bind_address when using with docker (#8526)

Signed-off-by: Christopher May-Townsend <chris@maytownsend.co.uk>

changelog.d/8526.doc

+Added note about docker in manhole.md regarding which ip address to bind to. Contributed by @Maquis196.

docs/manhole.md

@@ -5,22 +5,54 @@ The "manhole" allows server administrators to access a Python shell on a running
 Synapse installation. This is a very powerful mechanism for administration and
 debugging.
 
+**_Security Warning_**
+
+Note that this will give administrative access to synapse to **all users** with
+shell access to the server. It should therefore **not** be enabled in
+environments where untrusted users have shell access.
+
+***
+
 To enable it, first uncomment the `manhole` listener configuration in
-`homeserver.yaml`:
+`homeserver.yaml`. The configuration is slightly different if you're using docker.
+
+#### Docker config
+
+If you are using Docker, set `bind_addresses` to `['0.0.0.0']` as shown:
 
 ```yaml
 listeners:
   - port: 9000
-    bind_addresses: ['::1', '127.0.0.1']
+    bind_addresses: ['0.0.0.0']
     type: manhole
 ```
 
-(`bind_addresses` in the above is important: it ensures that access to the
-manhole is only possible for local users).
+When using `docker run` to start the server, you will then need to change the command to the following to include the
+`manhole` port forwarding. The `-p 127.0.0.1:9000:9000` below is important: it 
+ensures that access to the `manhole` is only possible for local users.
 
-Note that this will give administrative access to synapse to **all users** with
-shell access to the server. It should therefore **not** be enabled in
-environments where untrusted users have shell access.
+```bash
+docker run -d --name synapse \
+    --mount type=volume,src=synapse-data,dst=/data \
+    -p 8008:8008 \
+    -p 127.0.0.1:9000:9000 \
+    matrixdotorg/synapse:latest
+```
+
+#### Native config
+
+If you are not using docker, set `bind_addresses` to `['::1', '127.0.0.1']` as shown.
+The `bind_addresses` in the example below is important: it ensures that access to the
+`manhole` is only possible for local users).
+
+```yaml
+listeners:
+  - port: 9000
+    bind_addresses: ['::1', '127.0.0.1']
+    type: manhole
+```
+
+#### Accessing synapse manhole
 
 Then restart synapse, and point an ssh client at port 9000 on localhost, using
 the username `matrix`: