Prefer IPv4 for Postfix All Docker containers have an IPv4 address, they may not all have IPv6. This reduces potential transient errors in Postfix and still supports IPv6 only sites.
Prefer IPv4 for Postfix
All Docker containers have an IPv4 address, they may not all have IPv6.
This reduces potential transient errors in Postfix and still supports
IPv6 only sites.

JasonRivers/Docker-Nagios,drAlberT/docker-nagios