Add PGSQL support

[davidterranova]

Hi,
While it is mentioned in the README that you can run the image with PostGreSQL support, the binary does not actually support it. Here is the error message:

421 Unknown authentication method: pgsql:/etc/pureftpd/pgsql.conf

Which means that the binary is not built with the required extension.

Best regards

[gimoh]

The idea was that the package should be very minimal, which is what I needed for my use case.

I opened a request for another pure-ftpd package including PGSQL support.

I probably did get ahead of myself with the README though 😜
I'll add a note about some of the options being unavailable in this pure-ftpd build.