Testing upgrades on a backup system

Like many Debian users, I am planning very soon to upgrade to my own personal server ada to Debian Stretch.
Since I do a full rsync backup of the server to a different location, I was wondering if it was possible to use this backup root fs in a container to test the upgrade. Turns is out it works very well with systemd-nspawn

Copy the backup to a separate dir in /var/lib/machines

# /var/lib/machines is where machinectl look for images
# see https://superuser.com/a/307542 for rsync flags detial
rsync -axHAX --info=progress2 --numeric-ids --whole-file /home/manu/Projects/backups/ada_rootfs/ /var/lib/machines/ada/

For networking this container, I installed libvirtd, which comes with a DHCP server serving private IPs on the virbr0 bridge.

systemctl status libvirtd
libvirtd.service - Virtualization daemon
   Loaded: loaded (/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2017-06-08 23:15:33 CEST; 6 days ago
     Docs: man:libvirtd(8)
           http://libvirt.org
 Main PID: 2609 (libvirtd)
    Tasks: 18 (limit: 4915)
   Memory: 53.0M
      CPU: 2.448s
   CGroup: /system.slice/libvirtd.service
           ├─2609 /usr/sbin/libvirtd
           ├─4868 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile
           └─4869 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile

juin 09 11:34:34 django dnsmasq-dhcp[4868]: not giving name ada to the DHCP lease of 192.168.122.
juin 09 12:04:24 django dnsmasq-dhcp[4868]: DHCPREQUEST(virbr0) 192.168.122.17 06:91:4f:92:3b:a8
juin 09 12:04:24 django dnsmasq-dhcp[4868]: DHCPACK(virbr0) 192.168.122.17 06:91:4f:92:3b:a8 ada

Start the container, giving a a Virtual Ethernet Link connected to virbr0

systemd-nspawn --directory /var/lib/machines/ada --machine ada --network-bridge=virbr0 --boot --network-veth

(observe a nice booting sequence, up to a login prompt)
Login inside the container and configure the network

# snippet for /etc/network/interfaces
# the Virtual Ethernet Link appears as host0 inside the container
auto host0
iface host0 inet dhcp

# restart the network
systemctl restart networking

Find out from the host side which IP was assigned

machinectl list
MACHINE CLASS     SERVICE        OS     VERSION ADDRESSES
ada     container systemd-nspawn debian 8       192.168.122.17...

1 machines listed.

Inspect from the host system if the Apache virtual hosts are running

curl --silent --header 'Host: meinoesterreich.ist-ur.org' http://192.168.122.17/titelblatt | w3m -T text/html -dump

Happy Upgrades !