.customize_environment was failing and forcing me to boot as root (safe mode).
I've selectively recreated but even a simple apt-get update gets a bunch of errors like...
Err:1 http://packages.cloud.google.com/apt gcsfuse-buster InRelease Temporary failure resolving 'packages.cloud.google.com'
Not sure if this is the root cause, I'm ultimately trying to..
apt-get install -y libxss1
apt-get install -y libgbm-dev
apt-get install -y parallel
In the meantime I'm having to manually run a whole bunch of installs which is getting pretty repetitive.