Add Docker Image in Documentation

[vlomonaco]

Add Docker Hub avalanche image reference in the Readme and Gitbook.

[ggraffieti]

We have the github package! But we should also link the DockerHub image, maybe in the "current release" section of the readme?

[lrzpellegrini]

We already have a nightly Docker image which is also updated using the CI. It would be a waste not to add a reference to it in the GitBook doc.

What do you think of this @AntonioCarta?

[AntonioCarta]

Right now we provide a docker image only for the CI (using the nightly). If we want to provide docker images we should support at least different versions and properly tag them. IMO it's more effort than it's worth but I'm not against it if someone wants to support this long term (i.e. update the images whenever we have a new release).

Also, I would expect most people to start from a NVIDIA/pytorch image, and then adding their dependencies (like Avalanche). I don't see a use case for Avalanche-specific images.