Right, but what you're describing as running a pre-systemd "service" is really just "run a shell script that sources some environment files and then double forks".

Running a systemd service means talking to the /org/freedesktop/systemd1 object on the system bus typically listening on /run/dbus/system_bus_socket and asking it to start your service. This is all that systemctl really does.

    mkdir ./rootfs
    curl -O  https://cloud-images.ubuntu.com/minimal/releases/bionic/release/ubuntu-18.04-minimal-cloudimg-amd64-root.tar.xz | tar -xJPC ./rootfs
    mount -o bind /proc ./rootfs/proc
    mount -o bind /sys ./rootfs/sys
    mkdir ./rootfs/run/dbus
    mkdir ./rootfs/run/systemd
    mount -o bind /run/dbus ./rootfs/dbus
    mount -o bind /run/systemd ./rootfs/systemd
    chroot ./rootfs bash
    # See that it works just by sending dbus messages.
    dbus-send --system --print-reply --type=method_call --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnits
    # Now do the same with systemctl.
    export SYSTEMD_IGNORE_CHROOT=true
    systemctl list-units

THIS IS ALMOST CERTAINLY NOT WHAT YOU WANT. You're just talking to the host systemd. You won't see any of the services in your chroot since how could systemd know about them? Your chrooted root is also now just root on the host. Just use systemd-nspawn.

    # undo all the bind mount junk.
    chroot ./rootfs bash
    useradd ubuntu -G sudo
    passwd ubuntu
    exit
    cd ./rootfs
    systemd-nspawn -x --private-network --boot
    # login
    sudo systemctl list-units

This was very informative, thank you.