Add documentation about how to update the rust version

[andreeaflorescu]

This is an activity that is typically done by either me or @lauralt. To make it easier for other people to also update the Rust version we should better document the process. One of the limitations that we have is that updating the Rust version translated into publishing a new docker container. To publish a new docker container though you need to be part of the rust-vmm Docker org (which is a free plan org), which only has 3 seats available in total. These seats are currently occupied by @andreeaflorescu @aghecenco and @lauralt. We should find a way for the docker release to be automatic and not require our direct input. Instead, we should allow all maintainers to publish a container. See if this can be achieved with GitHub workflows.

[rbradford]

@andreeaflorescu We do this in CH: https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/.github/workflows/docker-image.yaml

[andreeaflorescu]

Thanks for the pointer @rbradford. Do you also use the manifest feature? I was wondering how I can treat that case because basically we need to publish the x86_64 tag, then the aarch64 tag, and then create the manifest which ends up being the actual version of the container we want to publish.

[rbradford]

@andreeaflorescu I think this must use the manifest feature as we build both architectures.