Add a shorter ARG to improve the experience

[initdc]

I add a shorter arg azure-cn for the --mirror option

[thaJeztah]

Thanks for contributing; I don't think we want to add more options to this list; the original list of mirrors were part of some partnerships (but those ended, so we may even consider removing those).

If you need to use a custom / alternative mirror, I would recommend using the DOWNLOAD_URL env-var when executing the script.