After a long long trial and error, the issue has resolved by adjusting systemd-networkd config. I used it to configure DHCP for the ethernet adapter. However, I did it with this config file /etc/systemd/network/20-local-wired.network:
[Match]
Name=*
[Network]
DHCP=yes
Although this was a heat-of-the-moment quickie to simplify the netplan/networkmanager mess, worked all right until docker’s fancy per-container interfaces appeared.
I don’t know why, but systemd-networkd somehow silently and weirdly breaks docker with the above config.
I’ve changed the match to en*, like this:
[Match]
Name=en*
[Network]
DHCP=yes
and suddently docker started working by the book.
Leaving this here for future google hits. 
Thanks for thinking along and being my rubber duck today!