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>

README.md

@@ -169,26 +169,31 @@ nix run github:kevinveenbirkenbach/package-manager#pkgmgr -- --help
 
 PKGMGR can be executed **inside Docker containers** for CI, testing and isolated
 workflows.
+
 ---
 
-#### Container types
+#### Container images
 
-Two container types are available.
+One image per distribution, each installing PKGMGR as a distro-native package
+on top of the matching base image from
+[base-images](https://github.com/kevinveenbirkenbach/base-images). Every image
+is a manifest list covering `linux/amd64` and `linux/arm64`.
 
+All images live under `ghcr.io/kevinveenbirkenbach/`:
 
-| Image type | Contains                      | Typical use             |
-| ---------- | ----------------------------- | ----------------------- |
-| **Virgin** | Base OS + system dependencies | Clean test environments |
-| **Stable** | PKGMGR + Nix (flakes enabled) | Ready-to-use workflows  |
+| Tag                         | Contains                           |
+| --------------------------- | ---------------------------------- |
+| `pkgmgr-<distro>:latest`    | newest build for that distribution |
+| `pkgmgr-<distro>:<version>` | one release                        |
+| `pkgmgr-<distro>:stable`    | newest release marked stable       |
 
-Example images:
+`<distro>` is one of `arch`, `debian`, `ubuntu`, `fedora`, `centos`. The Arch
+image is published without the suffix as well, so `pkgmgr:stable` and
+`pkgmgr-arch:stable` are the same image.
 
-* Virgin: `pkgmgr-arch-virgin`
-* Stable: `ghcr.io/kevinveenbirkenbach/pkgmgr:stable`
-
-
-Use **virgin images** for isolated test runs,
-use the **stable image** for fast, reproducible execution.
+For a clean environment that carries the system dependencies but no PKGMGR,
+pull `base-<distro>` from
+[base-images](https://github.com/kevinveenbirkenbach/base-images) directly.
 
 ---