So, according to the documentation, usernames must always include a
domain for SASL.
In other words. User cannot be `johhny` but `johhny@example.org`.
Further info can be found on this ticket: https://github.com/bokysan/docker-postfix/issues/192
This commit will automatically append domain if one is not provided in
`SMTPD_SASL_USERS`.
