pressure-vessel: Generate /etc/passwd, /etc/group for the container

• pressure-vessel: Generate /etc/passwd, /etc/group for the container

  Sometimes our current uid will not be present in /etc/passwd. For example, when using systemd-homed the equivalent of a passwd(5) record is synthesized by libnss_systemd. A similar thing could happen when using a remote user directory like LDAP, or for the group database.

  Inside the container, we have the basic nsswitch plugins from glibc (notably libnss_files, which reads /etc/passwd and /etc/group), but we do not have access to non-standard nsswitch plugins like libnss_systemd or LDAP. This can cause crashes or bugs in games that assume that getpwuid(getuid()) will always succeed (like Factorio), especially if they do not respect $HOME (like Factorio).

  Instead of bind-mounting the host system's /etc/passwd and /etc/group as-is, we can synthesize a /etc/passwd and /etc/group that are guaranteed to contain at least our uid and primary gid, matching what Flatpak does.

  Flatpak generates very minimal files that list our uid and gid, plus the default overflow uid and gid 65534 (labelled as nfsnobody for historical reasons). However, pressure-vessel has historically provided more complete files, so it's conceivable that a game might be relying on being able to resolve (for example) root as uid 0; so we copy everything from the host /etc/passwd and /etc/group, excluding only the line(s) that would duplicate the one we synthesize.

  Resolves: https://github.com/ValveSoftware/steam-runtime/issues/705

• tests: Exercise generation of /etc/passwd, /etc/group for container