# On LXC host
# lxc-stop -n lxc-testing
# qemu-img create -f raw vm-102-disk-1.raw 20G
# losetup /dev/loop0 vm-102-disk-1.raw 
# Create partition table (basically just start fdisk, press w and <enter>
# fdisk /dev/loop0
# mkfs.ext4 -F /dev/loop0
# mount /dev/loop0 /mnt/tmp/
# rsync -avP /var/lib/lxc/lxc-testing/rootfs/ /mnt/tmp/
# umount /mnt/tmp/
# losetup -d /dev/loop0
# rsync -vP --sparse vm-102-disk-1.raw pve:/mnt/lxc/images/102/

# On Proxmox host
# cd /etc/pve/lxc
# cp 101.conf 102.conf
# Edit config
# cat 102.conf 
arch: amd64
cores: 2
hostname: lxc-testing
memory: 2048
net0: name=eth0,bridge=vmbr0,hwaddr=FA:AA:BB:CC:AA:BB,ip=dhcp,ip6=dhcp,tag=41,type=veth
onboot: 1
ostype: debian
rootfs: ssd:102/vm-102-disk-1.raw,size=20G
swap: 2048