I tried to copy a folder from a remote machine:
acm@acm-docker2:/$ sudo rsync -avz firstname.lastname@example.org:/opt/symas /opt/
The authenticity of host ‘10.40.8.168 (10.40.8.168)’ can’t be established.
ECDSA key fingerprint is 38:0c:ee:74:49:65:7d:74:35:b3:a0:76:13:db:5e:74.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added ‘10.40.8.168’ (ECDSA) to the list of known hosts.
Permission denied, please try again.
..and failed because the root account is locked by default so I have to set a password first:
sudo passwd root
I ran the command again and this time it works.