Fix README to explain how to run docker from github packages instead of docker hub

[sgarciahelguera]

Our docker containers were moved from docker hub to github packages.
I don't know how to run docker from github packages in windows, someone with windows should pick this up, try to run docker on windows UI and then update the readme accordingly.
Maybe the way is to use docker compose instead of the windows UI, not sure. Anyway, the readme has to be updated.
Containers are here:
https://github.com/Cubeseed/Backend/pkgs/container/backend

To run from the command line the last container built for main branch:

docker run --name backend -dp 8000:8000 ghcr.io/cubeseed/backend:main

Containers are also uploaded for pull requests, e.g. for PR 44:

docker run --name backend -dp 8000:8000 ghcr.io/cubeseed/backend:pr-44