The best way is to disable password login and use SSH keys only. Any further steps are not required, but you may additionally install fail2ban or sshguard.
Your idea is correct, but I don't know how to do this in Wndows (while this is pretty simple in linux). However I want to warn you that if the partition that you are dumping is used by the OS, the resulting image will most likely be corrupted. Better use a linux live system and ensure that the partition is not mounted.
It asks for a path to a root directory of a bootstraped container. You can create it with debootstrap, rinse, pacstrap, alpine-chroot-install, virt-bootstrap etc.
Well, I don't use proxmox, however docker coexists with libvirt and other virtualization systems. If there are overlapping networks that docker ant proxmox attempt to manage, they are configurable.
Snapshotting in docker is as easy as docker commit. After that you can back it up with docker save. Then move to another host, but not without downtime.
However normally you need to backup/move only volumes attached to containers. If that's not the way how you like to organize your services, you likely don't need docker.
Camel book is the best technical book I ever read. It's unlikely that you'll program Perl, but it's really fun to read.