ci/docker: unify image build logic and run virgin tests across all distros

Refactor Dockerfile into multi-stage virgin/full targets and introduce a single
flag-based image build script. Standardize image naming, remove redundant build
scripts, and update Makefile targets accordingly. CI workflows now build missing
virgin images and run root and user tests consistently across all supported
distributions.

https://chatgpt.com/share/693c29d9-9b28-800f-a549-5661c783d968

Dockerfile

@@ -1,54 +1,57 @@
+# syntax=docker/dockerfile:1
+
 # ------------------------------------------------------------
-# Base image selector — overridden by Makefile
+# Base image selector — overridden by build args / Makefile
 # ------------------------------------------------------------
 ARG BASE_IMAGE
-FROM ${BASE_IMAGE}
 
-RUN echo "BASE_IMAGE=${BASE_IMAGE}" && \
-    cat /etc/os-release || true
+# ============================================================
+# Target: virgin
+# - installs distro deps (incl. make)
+# - no pkgmgr build
+# - no entrypoint
+# ============================================================
+FROM ${BASE_IMAGE} AS virgin
 
-# ------------------------------------------------------------
-# Nix environment defaults
-#
-# Nix itself is installed by your system packages (via init-nix.sh).
-# Here we only define default configuration options.
-# ------------------------------------------------------------
-ENV NIX_CONFIG="experimental-features = nix-command flakes"
+RUN echo "BASE_IMAGE=${BASE_IMAGE}" && cat /etc/os-release || true
 
-
-# ------------------------------------------------------------
-# Copy scripts and install distro dependencies
-# ------------------------------------------------------------
 WORKDIR /build
 
-# Copy only scripts first so dependency installation can run early
-COPY scripts/ scripts/
-RUN find scripts -type f -name '*.sh' -exec chmod +x {} \;
+# Copy scripts first so dependency installation can be cached
+COPY scripts/installation/ scripts/installation/
 
-# ------------------------------------------------------------
-# Select distro-specific Docker entrypoint
-# ------------------------------------------------------------
-# Docker entrypoint (distro-agnostic, nutzt package.sh)
-# ------------------------------------------------------------
-COPY scripts/docker/entry.sh /usr/local/bin/docker-entry.sh
-RUN chmod +x /usr/local/bin/docker-entry.sh
+# Install distro-specific build dependencies (including make)
+RUN bash scripts/installation/dependencies.sh
 
-# ------------------------------------------------------------
-# Build and install distro-native package-manager package
-# via Makefile `install` target
-# ------------------------------------------------------------
+# Virgin default
+CMD ["bash"]
+
+
+# ============================================================
+# Target: full
+# - inherits from virgin
+# - builds + installs pkgmgr
+# - sets entrypoint + default cmd
+# ============================================================
+FROM virgin AS full
+
+# Nix environment defaults (only config; nix itself comes from deps/install flow)
+ENV NIX_CONFIG="experimental-features = nix-command flakes"
+
+WORKDIR /build
+
+# Copy full repository for build
 COPY . .
-RUN find scripts -type f -name '*.sh' -exec chmod +x {} \;
 
-RUN set -e; \
-    echo "Building and installing package-manager via make install..."; \
-    make install; \
-    rm -rf /build
+# Build and install distro-native package-manager package
+RUN set -euo pipefail; \
+  echo "Building and installing package-manager via make install..."; \
+  make install; \
+  cd /; rm -rf /build
+
+# Entry point
+COPY scripts/docker/entry.sh /usr/local/bin/docker-entry.sh
 
-# ------------------------------------------------------------
-# Runtime working directory and dev entrypoint
-# ------------------------------------------------------------
 WORKDIR /src
-
 ENTRYPOINT ["/usr/local/bin/docker-entry.sh"]
 CMD ["pkgmgr", "--help"]