[docker][documentation] Review the rsync dependency in our Docker recipe and docs

This is related to #1372

Yesterday, while talking with @sparonuz, it was reported that the Autosubmit image did not work as expected on the EDITO Infra. One of the errors reported was a problem when rsync is not installed. Stella & Simon used our image, but extended it installing wget/zip/rsync.

wget/zip are not used directly in our code, I think. But rsync is (in migrate.py/autosubmit.py/paramiko_platform.py).

This issue is so that we confirm this is a system requirement, and if so (as I think it is) that it is installed in our Docker, and that our documentation has it stated clearly near the installation instructions.