I am alerting on boot events of my containers with a @reboot entry in crontab (this initiates sending an email).
Therefore, whenever the host is restarted I receive emails from all of my containers, as well as from the host itself. I would like to trim this down to only one email from the host, basically stating “everything is fine, I rebooted and therefore, all your containers rebooted as well, do not worry”.
How could I achieve such behavior? Initially, I tried to let the host somehow signal its shutdown intention to the containers, e.g., by touching a file inside of them (kind of a low-effort IPC). However, I cannot find the correct time to issue such a command. If I hook into the systemd shutdown sequence, the containers are likely already shut down. If I execute after the next boot, the containers are either not yet started or have already executed their reboot crontab entry.
I also researched a bit about identifying the cause of a reboot/shutdown from within a container. The only thing I could find is that systemd logs Received SIGRTMIN+3. when the host initiates a container shutdown. However, it seems complicated/error prone to search for this in the logs.
Is there any better way to achieve what I am trying?