systemctl does not work inside the toolbox

I'm setting up a toolbox for a development environment for a web application that requires postgresql running. I'm installing postgresql-server on the toolbox and then trying to start it using the usual method:

```
INS mildred@toolbox$ sudo systemctl start postgresql.service
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down
```

This is understandable as I'm running within a container, and systemd usually is not started in containers. Still, it means I have to manually start up postgresql by reading the systemd service file.

It would be great if systemd was started as entrypoint in the toolbox image, instead of `sleep +Inf`. Of course, udev and low level services would have to be prevented from starting so that a blank toolbox would not start any service.