bug#37162: ‘guix pack -f docker’ creates an image without /etc/passwd

[Ludovic Courtès]

‘guix pack -f docker’ currently creates an image without
/etc/{passwd,group,shadow}.

It’s OK most of the time, but again it looks like a gratuitous annoyance
for those cases where having them around matters (that’s also the reason
why guix-daemon creates them.)

Unless there are objections, I’d like to create these with just the
“root” and “nobody” accounts.  Or should we have a regular unprivileged
account?  But then what should its UID be?

Ludo’.