Address DOI comments about /etc/passwd

chgrp root /etc/passwd && chmod ug+rw /etc/passwd

Wider permissions on /etc/passwd is concerning. What use case is broken if the running user id doesn't exist?

echo ... >> /etc/passwd

Having the entrypoint itself modify /etc/passwd is fragile. Are there features that are broken if the user doesn't exist in /etc/passwd (like PostgreSQL's initdb that refuses to run)? Minimally, this should probably use useradd and usermod rather than hand editing.