docker-mailserver: Daily error-mail

I daily get emails which look either like this

/etc/cron.daily/logrotate: 
invoke-rc.d: WARNING: No init system and policy-rc.d missing! Defaulting to block.

or this:

/etc/cron.daily/logrotate: 
pkill: pidfile not valid 
Try `pkill --help' for more information. 
invoke-rc.d: WARNING: No init system and policy-rc.d missing! Defaulting to block. 
invoke-rc.d: WARNING: No init system and policy-rc.d missing! Defaulting to block.

My config: