Add to Wiki how to add a new user to the test email server

[DenBond7]

To add a new user to the test email server please follow these steps:

1. Go to docker-mailserver/docker-compose.yml. Uncomment #- ./maildata_volume:/var/mail
   
2. Run the local email server via docker-mailserver/run_email_server.sh
3. Run docker-mailserver/scripts/add_user.sh to add a new user. For example ./add_user.sh has_msgs_no_backups@flowcrypt.test
4. Check docker-mailserver/config/postfix-accounts.cf. There should be a new account.
   
5. After that it would be nice to log in with created credentials to prepare the user's folders on the email server. I use Thunderbird for that.
   
6. Then run docker-mailserver/stop_email_server.sh to stop the email server.
7. Go to docker-mailserver/docker-compose.yml. Comment - ./maildata_volume:/var/mail to disable storing data locally.
8. Copy docker-mailserver/maildata_volume/flowcrypt.test to docker-mailserver/maildata_source/flowcrypt.test. maildata_source will be used to add changes to a docker image.
9. After that need to update the docker image to apply changes.

It would be great to add it to Wiki

Originally posted by @DenBond7 in #1421 (comment)