Quiet the useradd and Perl warnings in the image build (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.

minsysusers, which runit's install runs to create its _runit-log user,
prints a Perl warning because runit's sysusers line leaves out the
shell. It reads /etc/sysusers.d/runit.conf in place of runit's file, so
the image writes a copy of that line there, naming the shell minsysusers
gives anyway; the user it creates is unchanged.

The runsvinit warning stays, since it needs a change to runsvinit: its
reaper and its own wait on runsvdir race for the same exited process.

Model: opus-5-5

Dockerfile

@@ -79,6 +79,15 @@ 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