Add /etc/passwd entry so ssh works in the spindle container
Setting HOME alone wasn't enough: OpenSSH calls getpwuid() up front
and aborts when no entry exists for uid 0, regardless of HOME. The
Nixery base image ships without one, so append a minimal root entry
before invoking ssh.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>