fix invalid --umask arg config
$ docker run -it --rm fphammerle/pyftpd-sink pyftpd-sink
usage: pyftpd-sink [-h] [--root path] [--user username]
[--pwd-hash sha256_hexdigest] [--ctrl-port port]
[--pasv-port port] [--pasv-addr ip_address] [--umask UMASK]
[--log-level level_name]
pyftpd-sink: error: argument --umask: invalid int value: ' default: env var $UMASK'