A Postfix container only relays virtual address.
If you have some internet domains and then many mail addresses, You may want to handle mails by one Gmail mailbox.
MX records point this container.
mkdir conf
vi conf/relay.conf
docker run --net=host -p 25:25 -v $PWD/conf:/ext -d pengo/relay-mailalice***@gmail.com = alice@example.net me@alice.example.jp
bob***@gmail.com = bar@example.net info@bar.example.org
: