cosign 3.0.0 not supported

The docker template does not support cosign versions greater or equal to 3.0.0.

Let's add this info in the README.md.

We can also make the docker-publish job fail if the signature cannot be copied (cf https://gitlab.com/to-be-continuous/docker/-/blob/8.0.1/templates/gitlab-ci-docker.yml?ref_type=tags#L1425)