This might be a little detailed for most, feel free to don your peril-sensitive sunglasses.

So, no offence to Debian 9.6 Stretch, but the rest of the fleet runs Ubuntu, which is very similar, but, well, some packages are different.

So lets see how we can make the Chromebook run a Ubuntu image and still have Wayland support and file sharing. There are a bunch of pages (cros-*) that add these things 'sommelier', 'garcon', 'wayland', etc. When your image is loaded in lxc, a /dev/.ssh mount is loaded which contains well-known keys for the username that matches the name of your *first* login account. For me, 'db'. I think you are kind of stuck with this name. The ssh keys are used for 'sshfs' which is how the Files app gets to your home dir within the container.

Now, lets try building a container that matches expectations.

lxc image copy ubuntu:18.04 local: --alias bionic
lxc launch bionic cros

lxc exec cros -- bash

Now we are in the being-prepped container, run:

echo "deb https://storage.googleapis.com/cros-packages stretch main" > /etc/apt/sources.list.d/cros.list
if [ -f /dev/.cros_milestone ]; then sudo sed -i "s?packages?packages/$(cat /dev/.cros_milestone)?" /etc/apt/sources.list.d/cros.list; fi
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551
apt update
apt install -y binutils adwaita-icon-theme-full 

apt download cros-ui-config
ar x cros-ui-config_0.12_all.deb data.tar.gz
gunzip data.tar.gz
tar f data.tar --delete ./etc/gtk-3.0/settings.ini
gzip data.tar
ar r cros-ui-config_0.12_all.deb data.tar.gz
rm -rf data.tar.gz

mkdir -p /opt/google/cros-containers/bin/sommelier
mkdir -p /opt/google/cros-containers/lib/
apt install -y libgl1-mesa-dri
cp /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so /opt/google/cros-containers/lib/

apt install -y cros-adapta cros-apt-config cros-garcon cros-guest-tools cros-sftp cros-sommelier cros-sommelier-config cros-sudo-config cros-systemd-overrides ./cros-ui-config_0.12_all.deb cros-unattended-upgrades cros-wayland
rm -rf cros-ui-config_0.12_all.deb
sed -i 's/Ambiance/CrosAdapta/' /etc/gtk-3.0/settings.ini
sed -i 's/ubuntu-mono-dark/CrosAdapta/' /etc/gtk-3.0/settings.ini
sed -i 's/gtk-sound-theme-name = ubuntu/gtk-font-name = Roboto 11/' /etc/gtk-3.0/settings.ini
sed -i '5d' /etc/gtk-3.0/settings.ini
sed -i -n '2{h;n;G};p' /etc/gtk-3.0/settings.ini

echo penguin > /etc/hostname

killall -u ubuntu
groupmod -n db ubuntu
usermod -md /home/db -l db ubuntu
usermod -aG users db
loginctl enable-linger db
sed -i 's/ubuntu/db/' /etc/sudoers.d/90-cloud-init-users
shutdown -h now

Now we are back to the host, run:

lxc publish cros --alias cros
lxc image export cros cros

Now, manually, put on USB, move to chromebook, copy to the default 'penguin' using the Files app

Now from the Termina:

lxc file pull penguin/home/db/cros.tar.gz $LXD_CONF

lxc stop --force penguin
lxc rename penguin google

lxc image import $LXD_CONF/cros.tar.gz --alias cros
lxc init cros penguin

OK we are done. And it worked. I now have a Ubuntu 18.04 image running, with file-sharing, and wayland for X-like stuff. I installed 'rxvt-unicode', and added a .Xdefaults file with a suitably large font (34) to overcome the DPI.