Some improvments

[thib3113]

Hi, I've done some improvments to this docker image .

• allow multiple accounts
• allow nearly all configurations from msmtp (works on all tested configurations)
• allow accounts to inherit from others
• allow to ask authentication on msmtpd
• don't write password to configuration file, keep it in secret

I keep the script backward compatible (and so I forbide some accounts names), it will add a deprecated message, and migrate the configuration to the account default

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Some notes :

• multiple accounts allow to deploy the same container for multiple applications
• all configurations can fix issue #53 because you can now set SMTP_<account>_LOGFILE=/var/log/msmtp
• inherit is usefull because account choice is based on the From header on the email ( -f %F par in the command from msmtpd ), so you can create one configuration, and inherit with multiple from
• authentication on msmtpd => allow to protect the endpoint, without giving a password to all of our apps
• don't write password => msmtp allow to pass a function to get the password, so, allowing to cat the docker secrets, without copying the password on another file

[thib3113]

Ping @crazy-max ? If you have some time for a feedback