/usr/share/unattended-upgrades/unattended-upgrade-shutdown always running, uses too much memory

[Steve Langasek]

On a default idle Ubuntu kinetic container, the second largest process by resident memory is '/usr/share/unattended-upgrades/unattended-upgrade-shutdown --wait-for-signal'.

We want unattended-upgrades to be installed by default in all container images, but this process is heavyweight and needs to go.

I see two possible solutions:
- construct a systemd unit that DTRT at shutdown without having to leave a process resident (but er the u-u maintainer knows a bit about systemd so this may not be feasible with the desired semantics?)
- instead of running the python script, implement a small shell wrapper that waits for the signal and execs u-u-shutdown, so that we only pay the memory cost of this script during shutdown.