Improvements in rootfs creation

[Ali-Nasrolahi]

This issue relates to improvements for create-rootfs, suggested during the transition from mmdebstrap to mkosi.
For context, see the discussion in #19.
Below is a compiled version of the points made in that discussion.

Please also mention:

• Anything that might have been missed.
• Any misinterpretation of a previous suggestion.
• Any unnecessary items in the list below.

Improvement Checklist

• Replace manual hostname setup with the --hostname option.
• Inject files using --extra-tree or --postinst-script.
  • Use --extra-tree for static files like hostname, resolv.conf, fstab, etc.
  • Use --postinst-script for setup steps like tweaking services.
  • This could remove the need for mount/bind steps and root privileges.
  • Also might avoid the current bug where shell variable references (e.g. $HOME, $PS1) get expanded to host values during image creation
• Automate SSH setup — check whether --ssh=true can replace the manual key copy.
• Would be nice to allow users to customize the mkosi invocation with extra or overridden options.
• Maybe consider using mkosi configuration files instead of long inline arguments for better versatility and readability.

Regards,
Ali