FYI, I’ve discovered my “Aborted Connection” was related to my UDM (Ultimate Dream Machine) “Endpoint Scanner”, part of Unifi’s Threat Management system that looks for suspicious open ports.
I appreciate the guide @nickrout . I installed MariaDB on my QNap and was able to get HA connected up to that DB with no issues. 
I have been meaning to come back to this thread to point out that when I reboot that machine, the mariadb docker container gets a different IP address and I need to xhange the secrets file. I am phasing out that machine in favour of a supervised setup. I also rarely reboot it.
You should be able to fix that by declaring the desired IP when specifying the exposed port:
-p 172.17.0.2:3306:3306
Default it is set to 0.0.0.0