chore: drop what the base-images refactor left behind

scripts/docker/slim.sh survived the removal of the stage that copied and ran
it, and nothing else referenced it.

The README still offered a `Virgin` image type with `pkgmgr-arch-virgin` as its
example, which is no longer published. Its second row was wrong independently
of that refactor: the image does not contain Nix, it runs `make install`, which
is scripts/installation/init.sh, and neither that nor entry.sh installs it. The
section now states the tags compute_publish_tags() actually pushes, including
the unsuffixed alias the default distribution gets, and points at base-images
for the clean environment the virgin image used to serve. A missing blank line
before a horizontal rule had also been rendering the paragraph above it as a
heading.

Three comment blocks explained what the repository used to do rather than what
it does, in the Dockerfile, in scripts/build/publish.sh and in the Makefile.
That belongs in the history, which is where it already is.

Verified: markdownlint-cli2 on the README goes from 21 findings to 10, all
remaining ones pre-existing and outside this section; `bash -n` on publish.sh
and `make -n build` pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,9 +1,5 @@
 # syntax=docker/dockerfile:1
 
-# One image, built on the distribution base published by
-# https://github.com/kevinveenbirkenbach/base-images, which owns the build
-# dependencies this used to install in a `virgin` stage of its own.
-#
 # hadolint ignore=DL3006,InvalidDefaultArgInFrom
 # BASE_IMAGE carries no default on purpose: a default would build one
 # distribution's image under another distribution's tag whenever the build arg