Workers: report real disk size (and CPU / RAM)

[benoit74]

Currently, disk size assigned to each workers is not much used AFAIK.

However, the disk size configured in zimfarm.config is displayed in the UI.

Would be great to at least check if the disk size available for docker containers is in accordance with what is configured in zimfarm.config, so that Zimfarm editors / devs are not mislead by a wrong configuration

By default, a df /var/lib/docker/overlay2 should do the trick, but this can be customized, don't know yet how to correctly check it)

Probably true for RAM / CPU as well, this would help diagnose problems as we have more and more workers which are not under our control.