FTP_USER entry now appended to virtual_users.txt instead of overwriting the whole file

[gregory112]

Check the virtual_users.txt file for FTP_USER entry first, to see if the entry already exists. If the user already exists, it won't be appended to the file. Using ">>" instead of ">" stream which will overwrite the file.

[evantill]

+1 for this PR

[fauria]

Thanks for the PR, @gregory112 !

We can try to find a way for appending virtual users, but the proposed grep approach wont work, as it will behave in an unexpected way if it matches a password.

[yale8848]

fix this issue here: https://github.com/yale8848/docker-vsftpd