My fix worked on my pi3b+ the night before I posted.
I'm trying to setup the docker plugin on a separate pi3b today and am getting the same rpc errors. I seems as though something has changed with the buster php packages or omv since my original post.
Warning: I take no responsibility for this making your system unstable, but this work-around worked for me...
sudo nano /etc/apt/sources.list
change:
deb http://httpredir.debian.org/debian stretch main contrib non-free
to:
deb http://httpredir.debian.org/debian buster main contrib non-free
sudo apt-get update
sudo apt-get install php-curl
It appears as though the install process is looking for dependencies in php7.0 (7.0.30-0+deb9ul), but the highest you can currently update php7.0 right now is (7.0.27-0+deb9ul). You might be able to get the docker GUI plugin to install if you update your /etc/apt/sources.list to include packages from the next release "buster" in order to manually install (7.0.30-0+deb9ul).
