Making /etc/passwd group writable allows privilege escalation

[jeblair]

This repository recommends making /etc/passwd group writable so that the uid_entrypoint script can add a user. At the very least, the uid_entrypoint script should end by removing the group write bit on the file, otherwise it could be written again to allow privilege escalation with su.

However, creating an image with a writeable /etc/passwd and relying on the entrypoint script to secure it means that any user who overrides the entrypoint in a container would lose that protection. Users do not generally expect that overriding an entrypoint will leave their container less secure. Therefore, while I believe this would be an improvement over the status quo, it is still flawed.

In general I am concerned that running as GID 0 may create similar unexpected vulnerabilities in cases where authors of other files on the system may have left them group-writable under GID 0 while not expecting a non-root user to be running with GID 0.