Quiet the update-alternatives man page warnings in the image build (closes #222) (#226)

xz-utils, which nix-bin depends on, links its man pages with update-alternatives when it installs, and the image's dpkg config drops man pages, so each link printed a warning. A config file re-includes only the man1 pages for this install; the same step deletes them, their dangling links in /etc/alternatives and the config file. No man page or package is added or removed.

Model: deepseek-v4-pro

Dockerfile

@@ -136,10 +136,19 @@ RUN mkdir /etc/sysusers.d \
 # ca-certificates, nix-bin and runit, from Ubuntu's archive. apt fetches
 # them over plain HTTP and checks each one against the archive's signed
 # package lists, so no CA certificates are needed.
-RUN apt-get update \
+#
+# 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/*
+    && rm -rf /var/lib/apt/lists/* /etc/dpkg/dpkg.cfg.d/z-man \
+    && 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.