fix: Prevent `stderr` being written to `/etc/postfix/main.cf` (#4147)

`stderr` is filtered by `grep` to discard unwanted (expected) log noise when appending the override `postfix-main.cf` content (_updated settings did not replace earlier defined instances_).

That `grep` filter introduced a regression into DMS v14 release, since any other `stderr` content not being excluded was now blended into `stdout` and redirected with the original `stdout` output for the `postconf -n` command.

The fix is to ensure the `grep` output is redirect to `stderr` to avoid that mishap.

CHANGELOG.md

@@ -31,6 +31,8 @@ All notable changes to this project will be documented in this file. The format
 - **Dovecot:**
   - Update logwatch `ignore.conf` to exclude Xapian messages about pending documents ([#4060](https://github.com/docker-mailserver/docker-mailserver/pull/4060))
   - `dovecot-fts-xapian` plugin was updated to `1.7.13`, fixing a regression with indexing ([#4095](https://github.com/docker-mailserver/docker-mailserver/pull/4095))
+- **Internal:**
+  - Fixed a regression introduced in v14 where `postfix-main.cf` appended `stderr` output into `/etc/postfix/main.cf`, causing Postfix startup to fail ([#4147](https://github.com/docker-mailserver/docker-mailserver/pull/4147))
 
 ### CI
 

target/scripts/startup/setup.d/postfix.sh

@@ -129,7 +129,7 @@ function __postfix__setup_override_configuration() {
     # Do not directly output to 'main.cf' as this causes a read-write-conflict.
     # `postconf` output is filtered to skip expected warnings regarding overrides:
     # https://github.com/docker-mailserver/docker-mailserver/pull/3880#discussion_r1510414576
-    postconf -n >/tmp/postfix-main-new.cf 2> >(grep -v 'overriding earlier entry')
+    postconf -n >/tmp/postfix-main-new.cf 2> >(grep -v 'overriding earlier entry' >&2)
 
     mv /tmp/postfix-main-new.cf /etc/postfix/main.cf
     _adjust_mtime_for_postfix_maincf