refactor(docker): consume the base images instead of building them

The `virgin` stage installed the distribution build dependencies, and that
layer now lives in its own repository:
https://github.com/kevinveenbirkenbach/base-images. What remains here is one
image per distribution that installs pkgmgr on top of it. `slim` goes with it:
it was published for every distribution and pulled by nothing.

scripts/build/base.sh stops pinning five upstream images and resolves
ghcr.io/<owner>/base-<distro>:<tag> instead. Its env names follow the images
repository rather than this one, so the code shows whose namespace that is.
image.sh loses the --target axis, the -virgin/-slim tag suffixes with it, and
passes --platform on push. Manjaro joins the set; os_resolver.sh already maps
it onto arch, so it needs no dependency script of its own.

The two virgin workflows pull the base image rather than building it. What
they prove is unchanged: pkgmgr installs into an untouched container, as root
and as an unprivileged user.

test_distro_dependency_scripts_install_gpg_tools goes: it grepped the
dependency scripts for gnupg, and those scripts are base-images' responsibility
now, where the contract test runs `gpg --version` instead of reading a package
list.

Verified against locally built base images: the arch image builds FROM
base-arch, and the full suite passes inside it - 405 unit, 90 integration and
49 e2e tests.

Requires base-images to be published first; until then every build here fails
at the pull.

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

Dockerfile

@@ -1,67 +1,29 @@
 # syntax=docker/dockerfile:1
 
-# ------------------------------------------------------------
-# Base image selector — overridden by build args / Makefile
-# ------------------------------------------------------------
+# 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
+# is forgotten. scripts/build/base.sh is the only place that resolves it.
 ARG BASE_IMAGE
+FROM ${BASE_IMAGE}
 
-# ============================================================
-# Target: virgin
-# - installs distro deps (incl. make)
-# - no pkgmgr build
-# - no entrypoint
-# ============================================================
-FROM ${BASE_IMAGE} AS virgin
 SHELL ["/bin/bash", "-lc"]
 
-RUN echo "BASE_IMAGE=${BASE_IMAGE}" && cat /etc/os-release || true
-
 WORKDIR /build
 
-# Copy scripts first so dependency installation can be cached
-COPY scripts/installation/ scripts/installation/
-
-# Install distro-specific build dependencies (including make)
-RUN bash scripts/installation/dependencies.sh
-
-# Virgin default
-CMD ["bash"]
-
-
-# ============================================================
-# Target: full
-# - inherits from virgin
-# - builds + installs pkgmgr
-# - sets entrypoint + default cmd
-# - NOTE: does NOT run slim.sh (that is done in slim stage)
-# ============================================================
-FROM virgin AS full
-
-WORKDIR /build
-
-# Copy full repository for build
 COPY . .
 
-# Build and install distro-native package-manager package
 RUN set -eu; \
   echo "Building and installing package-manager via make install..."; \
   make install; \
   rm -rf /build
 
-# Entry point
 COPY scripts/docker/entry.sh /usr/local/bin/docker-entry.sh
 
 WORKDIR /opt/src/pkgmgr
 ENTRYPOINT ["/usr/local/bin/docker-entry.sh"]
 CMD ["pkgmgr", "--help"]
-
-
-# ============================================================
-# Target: slim
-# - based on full
-# - runs slim.sh
-# ============================================================
-FROM full AS slim
-
-COPY scripts/docker/slim.sh /usr/local/bin/slim.sh
-RUN chmod +x /usr/local/bin/slim.sh && /usr/local/bin/slim.sh