Run nightly tasks on a random minute after 03:00 to avoid overload (#1754)

- The MIAB version check regularly fails at 03:00, presumably because a
  large portion of installations is checking mailinabox.email at the same
  time.
- At installation time, the time of the nightly clock is configured to
  run at a random minute after 03:00, but before 04:00.
- Users might expect the nightly tasks to be over at a certain time and
  run their own custom tasks afterwards. This could thus interfere with
  custom backup routines.
- This breaks reproducibility of the installation process.
- Users might also be surprised by the nightly task time changing after
  updating MIAB.

CHANGELOG.md

@@ -12,6 +12,7 @@ Software updates:
 
 System:
 
+* Nightly backups now occur on a random minute in the 3am hour (in the system time zone). The minute is chosen during Mail-in-a-Box installation/upgrade and remains the same until the next upgrade.
 * Fix for mail log statistics report on leap days.
 * Fix Mozilla autoconfig useGlobalPreferredServer setting.
 

setup/management.sh

@@ -101,10 +101,11 @@ hide_output systemctl enable mailinabox.service
 # Perform nightly tasks at 3am in system time: take a backup, run
 # status checks and email the administrator any changes.
 
+minute=$((RANDOM % 60))  # avoid overloading mailinabox.email
 cat > /etc/cron.d/mailinabox-nightly << EOF;
 # Mail-in-a-Box --- Do not edit / will be overwritten on update.
 # Run nightly tasks: backup, status checks.
-0 3 * * *	root	(cd `pwd` && management/daily_tasks.sh)
+$minute 3 * * *	root	(cd `pwd` && management/daily_tasks.sh)
 EOF
 
 # Start the management server.