pure-ftpd: Broken config path by default

[fwenzel]

The docker config demands pureftpd.conf (sic): https://github.com/vimagick/dockerfiles/blob/master/pure-ftpd/Dockerfile#L19

but the file name has a dash: https://github.com/vimagick/dockerfiles/blob/master/pure-ftpd/data/pure-ftpd/pure-ftpd.conf

[fwenzel]

PS: Thank you for your awesome docker files!