Systemd-nspawn seems like a viable Vagrant alternative

I’m working on a Python project where packaging container images is too uneconomical compared to just pushing source code, so I went with the latter, using an ansible-like configuration management system to manage a pet server. That also means I need some way to replicate the pet locally. Way back in my first job we used to utilize Vagrant for such a use case, but if memory serves, it was painfully slow and would have minor breakages all the time. To be honest it could have been either virtualbox or my inexperienced fresh graduate self’s fault, but I’m sure of one thing: initializing a Virtualbox-backed Vagrant VM is slooooow.

That leads us to… containers! Not the “application containers” category made mainstream by Docker and friends, but the old-school “system containers” that act like lightweight VMs: booting one will launch the full init system (read: systemd) which then spawns the main application along with any supporting services. Historically lxc/lxd (now incus) was the most notable in this space, but relatively recently systemd threw its hat in the ring with systemd-nspawn. I’ve dabbled with incus for a while but its configuration never clicked with me, and networking breaks if it’s run on the same machine with Docker. Unfortunately I still need to touch Docker from time to time, so that’s an major inconvenience. To be fair, this seems like Docker’s fault for its overzealous iptables configs, and there are workarounds, but I’m not comfortable with any of those. On the other hand, systemd-nspawn just works, albeit with some manual network configuration.

# On host, enable systemd-networkd and create a debian bookworm root dir:
systemctl enable --now systemd-networkd
debootstrap \
    --include=dbus,libpam-systemd,systemd-resolved \
    bookworm \
    /var/lib/machines/myguest \
    'https://mirror.bizflycloud.vn/debian/'

# Set root password on guest:
systemd-nspawn -D /var/lib/machines/myguest passwd

# Now spawn a guest shell and enable necessary network services:
systemd-nspawn -D /var/lib/machines/myguest --network-veth -b
#> systemctl enable --now systemd-networkd systemd-resolved