Ahhh, my fundamental understanding of docker has been flawed. Thank you in pointing me in the right direction. All I needed to do was to use the ubuntu address and NOT the container address! Everything works beautifully on my home network now. Regarding the docker-compose file, I am using exact copy taken from https://vikunja.io/docs/docker-walkthrough/
To access Vikunja from the internet
I now set up port forwarding on my router on port 80 to point to my Ubuntu server, and I can access it form the web perfectly well. I am however worried about security implications of this. Would you advise against this kind of set up?