docker-postfix/docker-compose.yml

version: '2'

services:
  postfix:
    build: .
    image: fphammerle/postfix
    environment:
      POSTFIX_RELAYHOST: relayhost.example.com:submission
      POSTFIX_RELAY_DOMAINS: example.com
    ports: ['127.0.0.1:25:25']
    volumes: [/dev/log]
    security_opt: [no-new-privileges]
    restart: unless-stopped

# https://docs.docker.com/compose/compose-file/compose-file-v2/