Add documentation about UID and GID access

[JayDi85]

Docker's ftp virtual user (FTP_USER_NAME) uses default UID:GID as 1000:1000. So that's ID maps to host machine's user with same IDs and must be prevelegy compatible. If not then access denied error possible (error 500) on login or files operations.

Two important things missing from documentation:

• 1000:1000 must have access to FTP_USER_HOME on host;
• 1000:1000 must have read/write access for all files/folders available to edit from FTP.

Problem example (related issues: #109, #51):

• Fresh ubuntu install;
• It created two users (root and custom -- the last one is 1000:1000);
• Switch to root mode and prepare docker config to run (create folders, yml, etc);
• Run it and work with FTP;
• Errors: you can't connect by ftp to home folder (no access for 1000);
• Errors: you can't delete or create files in prepared folders by root (no access for 1000).

[stilliard]

Hi @JayDi85

Thanks for reporting this, would you be interested in making a pull request to add this to the readme file under the existing "Setting runtime FTP user" section?

[bartagergely]

I would be very thankfull for a detailed guide for how to use this image with multiple ftp users with different homedirs. What are the correct UID settings on the host's directories (the homedirs), and what is the correct setting for UIDs inside the pureftpd.passwd file.