Host key verification failed for 'Transferring file from remote host with rsync to local host without rsync:'

[KuroAshi75]

We have two ubuntu host systems[ssh client and ssh server] with docker container running on the client. our aim is to transfer the file from server host to client host using the docker container[instrumentisto /rsync-ssh ]running on the client.
We have generated the public and private key on the client[id_rsa,id_rsa.pub] and added the public key in the authorized_keys file on the server. we have tried the following command and we're getting the "Host key verification failed" error.

sudo docker run --rm -i -v /home/testuser1/.ssh/:/ssh-key/ -v /home/testuser1/client_rsync/:/mnt/ instrumentisto/rsync-ssh rsync -auvz -e "ssh -i /ssh-key/id_rsa" testuser1@[ip address of server]:/home/testuser1/server_rsync/ /mnt/

Note: we have verified the keys by ssh login from the client to server and it logs in without password

[tyranron]

@KuroAshi75 make sure your private key has correct rights/owner inside container.