LFTP is useful when you don't have SSH access to a server but want rsync functionality on S/FTP basis.
Just prepend lftp command with docker run stefanso/lftp:
docker run --rm -i stefanso/lftp lftp --help
Copy folder from local to remote using docker volume
docker run --rm -i -v <path-to-local-folder>:/mnt stefanso/lftp lftp -c "open -u {$USER},{$PASSWORD} {$REMOTE_SERVER_HOSTNAME_OR_IP}; set ssl:verify-certificate no; mirror -R /mnt {$REMOTE_PATH}"