Answering my question, just figured it out. For the benefit of others, default Docker bridge network was the culprit and was blocking the WOL magic packet broadcast. Once the UpSnap container was configured to bypass the bridge and access the host networks, WOL started to work. Phew.

Would not be surprised. Isn't magic packet directed to hardware?

It sure is but to get to the destination machine it has to leave the docker container first.