[Docker] How to Set up Postfix Containers and Send Emails From WordPress in Docker

Add SMTP mail functionality to the official Docker WordPress image. See how to
– Install Postfix in a separate Docker container: 2 examples — with DKIM and without.
– Connect to these Postfix containers from the WordPress Docker container.
– Add Postfix to the Docker WordPress container with a Dockerfile.
– Set a PTR record for reverse DNS lookup.
– Set DNS records: SPF and DKIM.