Create the smallwebwaf user without useradd's uid warning (closes #58)

useradd --system warns when the uid it is given is above SYS_UID_MAX,
999 on Ubuntu; --key raises that limit for this one call, so the uid
stays 65532.

The issue's two other warnings stay, since each needs a change to an
outside tool: minsysusers, which runit's install runs to create its
_runit-log user, reads a shell field that runit's sysusers line leaves
out; runsvinit's reaper and its own wait on runsvdir race for the same
exited process.

Model: opus-5-5

Dockerfile

@@ -79,15 +79,6 @@ RUN git checkout --quiet --detach b4b2c785308b1ce785b6155c7fe5f16879080193 \
 # ubuntu 26.04, 2026-09-27
 FROM ubuntu@sha256:f144425ff09be612d6d9ad965196e9cdc23dae1f42110a8a11a3e9a8198759f7
 
-# runit's install creates its _runit-log user with minsysusers, which
-# reads this file in place of runit's /usr/lib/sysusers.d/runit.conf.
-# runit's line leaves out the shell, and minsysusers prints a Perl
-# warning for that; this copy of it names /sbin/nologin, the shell
-# minsysusers gives when none is named.
-RUN mkdir /etc/sysusers.d \
-    && echo 'u  _runit-log  -  "runit svlogd user"  /nonexistent  /sbin/nologin' \
-        > /etc/sysusers.d/runit.conf
-
 # ca-certificates, nix-bin and runit, from Ubuntu's archive as it was at
 # the snapshot moment, which is never earlier than the Ubuntu image above.
 # apt checks every package against the snapshot's InRelease files, and