tools (common/email): try to guess sender domain when necessary: when `DNSNAME` is `localhost.localdomain` and is `mail_sender_domain` is not set, a guess is done to find a proper domain, using `email_from`.

version: "3.6"
services:

  sat:
    image: salutatoi/sat
    build: backend

  libervia:
    image: salutatoi/libervia
    build: libervia
    depends_on:
      - sat