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

.github/workflows/test-virgin-user.yml

@@ -7,6 +7,10 @@ jobs:
   test-virgin-user:
     runs-on: ubuntu-latest
     timeout-minutes: 45
+    strategy:
+      fail-fast: false
+      matrix:
+        distro: [arch, debian, ubuntu, fedora, centos]
 
     steps:
       - name: Checkout repository
@@ -15,19 +19,24 @@ jobs:
       - name: Show Docker version
         run: docker version
 
-      - name: Virgin Arch pkgmgr user test (non-root with sudo)
+      # 🔹 BUILD virgin image if missing
+      - name: Build virgin container (${{ matrix.distro }})
+        run: |
+          set -euo pipefail
+          distro="${{ matrix.distro }}" make build-missing
+
+      # 🔹 RUN test inside virgin image as non-root
+      - name: Virgin ${{ matrix.distro }} pkgmgr test (user)
         run: |
           set -euo pipefail
 
           docker run --rm \
             -v "$PWD":/src \
             -w /src \
-            archlinux:latest \
+            "pkgmgr-${{ matrix.distro }}-virgin" \
             bash -lc '
               set -euo pipefail
 
-              pacman -Syu --noconfirm git python python-pip sudo base-devel debugedit nix make
-
               make install
 
               useradd -m dev
@@ -35,7 +44,6 @@ jobs:
               chmod 0440 /etc/sudoers.d/dev
               chown -R dev:dev /src
 
-              # --- make Nix usable for non-root inside this container ---
               mkdir -p /nix/store /nix/var/nix /nix/var/log/nix /nix/var/nix/profiles
               chown -R dev:dev /nix
               chmod 0755 /nix
@@ -45,21 +53,13 @@ jobs:
                 set -euo pipefail
                 cd /src
 
-                echo \">>> [dev] Using user: \$(whoami)\"
-
-                echo \">>> [dev] Running make setup-venv...\"
                 make setup-venv
-
-                echo \">>> [dev] Activating venv...\"
                 . \"\$HOME/.venvs/pkgmgr/bin/activate\"
 
-                echo \">>> [dev] Running: pkgmgr version pkgmgr\"
                 pkgmgr version pkgmgr
 
-                echo \">>> [dev] Running Nix-based pkgmgr version...\"
                 export NIX_REMOTE=local
                 export NIX_CONFIG=\"experimental-features = nix-command flakes\"
                 nix run /src#pkgmgr -- version pkgmgr
               "
             '
-