Add note to manhole.md about bind_address when using with docker

changelog.d/8526.doc

@@ -0,0 +1 @@
+Added note about docker in manhole.md regarding which ip address to bind to. Contributed by @Maquis196.

docs/manhole.md

@@ -18,6 +18,16 @@ listeners:
 (`bind_addresses` in the above is important: it ensures that access to the
 manhole is only possible for local users).
 
+**Docker Note**
+If you are using docker, please ensure you use the following example;
+
+```yaml
+listeners:
+  - port: 9000
+    bind_addresses: ['0.0.0.0']
+    type: manhole
+```
+
 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.