cannot get docker-ce to install (nanopi m4 V2; Armbian_20.08.1_Nanopim4_buster_current_5.8.6.img)

Hi

Trying to install docker (arm64) via the debian documentation on Docker's website, but running into persistant problem with dpkg on docker-ce. This is a fresh install (good  SD card) of Armbian_20.08.1_Nanopim4_buster_current_5.8.6.img. (Made with dd, I confess).

No ssh yet, so I'll handtype the some terminal output:

...
Job for docker.service failed because the control provess exited with error code.
See systemctl status docker.service" and "journalctl -xe" for details.
invoke-rc.d: initscript docker, action "start" failed.
- docker.service - Docker Application Container Engine
    Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
    Active: activating (auto-restart) (Result: exite-code) since #time stamp
...
Process: 15803 ExecStart=/usr/bin/dockerd -H fd:// --containerd=run/containerd/containerd.sock (code=exited, status=1/FAILURE)
Main PID: #tired of typing, just says it exited status=1
dpkg: error processing package docker-ce (--configure):
    installed docker-ce package post-installation script subprocesses returned error exit status 1
...

systemctl status docker.service show problems with restarting:

.

..    docker.service: start request repeated too quickly
    docker.service: Failed with result 'exit-code'
    Failed to start Docker Application Container Engine.

    
journalctl says:

    docker.socket: Failed with result 'service-start-limit-hit"

    
I have removed and purged (apt), deleted relevant files in /var/lib, cleaned and autoremoved, reinstalled all the recommended packages. I still just get the same.

Nothing quite the same in existing forum posts, but having read them I have also:

• run `ambianmonitor -u` output: http://ix.io/2CnN
• Tried the instructions here for a diagnosis with docker's check-config.sh. It suggested I need to install apparmor. I did: no difference.
• It also said I needed CONFIG_NF_NAT_IPV4 and CONFIG_NF_NAT_NEEDED. I don't know what to do about that. (It also suggested several optional items were missing, but I don't think they are relevant if they are optional).

Does anyone have some insights? Or - better - solutions?