If you’ve installed node-red-contrib-blynk-ws from the pallet manager and created a new connection to your Blynk Legacy local server and specified the correct port and it’s not working then I guess it’s going to be an issue with port mapping in your Docker.
I tried Docker once and hated it, so I don’t have any real knowledge of how to get around that issue.
My solution is to run Node-Red on a Raspberry Pi 4 with an external SSD rather than using an SD card. The SSD is far more robust and also faster, but backups are of course also a good idea 
I don’t really have a basic “getting started” tutorial, and if I did it wouldn’t cover Docker, but there’s some stuff buried in this topic…
Pete.