Quiet the update-alternatives man page warnings in the image build (closes #222)

nix-bin depends on xz-utils, whose install links man pages with
update-alternatives. The image's dpkg config drops man pages, so each link
warns. A config file, read after that one, re-includes the man1 pages for
this install alone; the cleanup then deletes them again, so no man page or
package is added.

Model: deepseek-v4-pro

Dockerfile

@@ -137,25 +137,18 @@ RUN mkdir /etc/sysusers.d \
 # them over plain HTTP and checks each one against the archive's signed
 # package lists, so no CA certificates are needed.
 #
-# The image's dpkg config drops man pages as each package unpacks, so
-# xz-utils' install, which nix-bin depends on, prints an update-alternatives
-# warning for every man-page link it cannot make. This side file re-includes
-# them for this install alone; it is read after /etc/dpkg/dpkg.cfg.d/excludes,
-# so it overrides the drop and the links are made quietly. The cleanup below
-# removes the man pages and the links and directories they added, then puts
-# the base image's empty man directories back. The image holds exactly what
-# it held before, and installs built from it still drop man pages.
-RUN find /usr/share/man -mindepth 1 -type d > /tmp/base-man-dirs \
-    && echo 'path-include=/usr/share/man/*' > /etc/dpkg/dpkg.cfg.d/z-man \
+# nix-bin depends on xz-utils, whose install links man pages with
+# update-alternatives. The image's dpkg config drops man pages, so each
+# link warns. z-man, read after /etc/dpkg/dpkg.cfg.d/excludes, re-includes
+# the man1 pages for this install alone; the same step deletes them again,
+# so no man page or package is added.
+RUN echo 'path-include=/usr/share/man/man1/*' > /etc/dpkg/dpkg.cfg.d/z-man \
     && apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install --yes --no-install-recommends \
         ca-certificates nix-bin runit \
     && rm -rf /var/lib/apt/lists/* /etc/dpkg/dpkg.cfg.d/z-man \
-    && find /usr/share/man ! -type d -delete \
-    && find /etc/alternatives -type l -xtype l -delete \
-    && find /usr/share/man -mindepth 1 -type d -empty -delete \
-    && xargs -a /tmp/base-man-dirs mkdir -p \
-    && rm /tmp/base-man-dirs
+    && find /usr/share/man/man1 ! -type d -delete \
+    && find /etc/alternatives -type l -xtype l -delete
 
 # Nix run by root expects a group of build users, which nix-bin does not
 # create; with the setting empty, root's builds run without them.