Add mailer service to simplify config

[spwoodcock]

Issue

• Currently we rely on external SMTP providers, such as gmail.
• This adds an extra step to the setup.

Solution

• Bundle a simple SMTP service that can be used to send email.
• Example (added as mail.yml compose file, to make it optional):

version: '3'
services:
  mail:
    image: "ixdotai/smtp:v0.5.2"
    volumes:
      - ./rsa.private:/etc/exim4/dkim.key.temp:ro
    environment:
      - MAILNAME=osmseed.org
      - DKIM_KEY_PATH=/etc/exim4/dkim.key.temp
    restart: unless-stopped

We can simply add an empty file rsa.private file somewhere in the repo to make this work (required by ixdotai/smtp).