In the newer controller versions, you should be able to go to
Settings -> NETWORK SETTINGS -> Device Authentication
To view the SSH credentials.
Go to Settings -> Site -> DEVICE AUTHENTICATION
Pull UniFi 5.6.42 docker image
sudo docker pull jacobalberty/unifi:5.6.42
Install image
sudo docker run -d --init --restart=unless-stopped --name=unifi-controller --net=host --volume=/docker/unifi:/var/lib/unifi -p 8080:8080/tcp -p 8081:8081/tcp -p 8443:8443/tcp -p 8843:8843/tcp -p 8880:8880/tcp -p 8883:8883/tcp -p 3478:3478/udp jacobalberty/unifi:5.6.42
Access and setup the unifi controller from a browser.
https://ip-address:8443