fix: Ensure main log file is tailed from the start (#4146)

Co-authored-by: Casper <casperklein@users.noreply.github.com>

CHANGELOG.md

@@ -32,6 +32,7 @@ All notable changes to this project will be documented in this file. The format
   - 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:**
+  - The main `mail.log` which is piped to stdout via `tail` now correctly begins from the first log line of the active container run. Previously some daemon logs and potential warnings/errors were omitted. ([#4146](https://github.com/docker-mailserver/docker-mailserver/pull/4146))
   - 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/start-mailserver.sh

@@ -194,10 +194,16 @@ fi
 # marker to check if container was restarted
 date >/CONTAINER_START
 
+# Container logs will receive updates from this log file:
+MAIN_LOGFILE=/var/log/mail/mail.log
+# NOTE: rsyslogd would usually create this later during `_start_daemons`, however it would already exist if the container was restarted.
+touch "${MAIN_LOGFILE}"
+# Ensure `tail` follows the correct position of the log file for this container start (new logs begin once `_start_daemons` is called)
+TAIL_START=$(( $(wc -l < "${MAIN_LOGFILE}") + 1 ))
+
 [[ ${LOG_LEVEL} =~ (debug|trace) ]] && print-environment
 _start_daemons
 
+# Container start-up scripts completed. `tail` will now pipe the log updates to stdout:
 _log 'info' "${HOSTNAME} is up and running"
-
-touch /var/log/mail/mail.log
-exec tail -Fn 0 /var/log/mail/mail.log
+exec tail -Fn "+${TAIL_START}" "${MAIN_LOGFILE}"