bug#38432: dockerd is not started automatically

[Maxim Cournoyer]

Hello Danny!

The service definition here:
https://github.com/containerd/containerd/blob/master/containerd.service
uses the systemd notify mechanism to allow the application (containerd)
to message the init (systemd) that it is ready.

leoprikler on #guix had the idea to patch this away and replace it by
some code that'd write a PID file.  That's an interesting idea!  Even
nicer would be to have Shepherd listen on the socket that the sd_notify
mechanism relies on and natively support the systemd notify thing :-).

Food for thoughts...

Maxim