Merge #3709

3709: Clarify ip listen address setting r=mergify[bot] a=nextgens

## What type of PR?

documentation

## What does this PR do?

Clarify ip listen address setting in setup to avoid open-relays.

Thanks to `@Cenness`  for reporting it and suggesting a better wording.

### Related issue(s)
- closes #3680
- closes #3683
- #3690

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [ ] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

setup/templates/steps/compose/03_expose.html

@@ -10,8 +10,8 @@ IP addresses in order to expose its public services. You must at least set up
 an IPv4 or an IPv6 address if you wish to access Mailu.</p>
 
 <p><span class="label label-warning">Warning</span> You must use specific addresses, please
-avoid generic all-interfaces addresses like <code>0.0.0.0</code> or <code>::</code>.
-<a href="https://mailu.io/{{ version }}/compose/setup.html#bind-address">How to find these addresses.</a></p>
+avoid generic all-interfaces addresses like <code>0.0.0.0</code> or <code>::</code>. If you still want to use generic addresses you <b>MUST</b> disable <code>userland-proxy</code>; failing to do so will turn your Mailu installation into an open-relay.
+<a href="https://mailu.io/{{ version }}/compose/setup.html#bind-address">How to find these addresses.</a> <a href="https://mailu.io/{{ version }}/faq.html#how-to-make-ipv6-work">How to disable userland-proxy.</a></p>
 
 <div class="form-group">
   <label>IPv4 listen address</label>

towncrier/newsfragments/3680.misc

@@ -0,0 +1 @@
+Modern docker versions enable the userland-proxy by default, clarify in setup that unless the bindto address is specific that will create problems.