The volume configuration is not required anymore, we have
a default config in /etc/sftpgo and the volume config is
mounted on /srv/sftpgo/config so we always have a default
config in /etc/sftpgo/sftpgo.json.
Anyway to persist server's keys after an image rebuild a
configuration volume is highly recommended
detect upload/download errors and don't execute actions if a transfer
error happen.
To detect SFTP errors this patch is needed:
https://github.com/pkg/sftp/pull/307