Improve Docker-based build & test pipeline for pkgmgr

- Add .dockerignore rules to prevent Arch package artifacts from entering the build context
- Rework Dockerfile to remove stale package artifacts before makepkg and use a dev entry script
- Introduce docker-entry-dev.sh to always rebuild pkgmgr from the mounted /src tree
- Update Makefile 'test' target to rebuild pkgmgr inside the container before running tests
- Fix predictable makepkg failures caused by residual *.pkg.tar.* files

Conversation reference: https://chatgpt.com/share/6935e6e8-f3fc-800f-a4e9-7537114f13d1

scripts/docker-entry-dev.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+echo "[entry] Using /src as working tree for package-manager..."
+cd /src
+
+# Optional: altes Paket entfernen
+echo "[entry] Removing existing 'package-manager' Arch package (if installed)..."
+pacman -Rns --noconfirm package-manager || true
+
+# Build-Owner richtig setzen (falls /src vom Host kommt)
+echo "[entry] Fixing ownership of /src for user 'builder'..."
+chown -R builder:builder /src
+
+echo "[entry] Rebuilding Arch package from /src as user 'builder'..."
+su builder -c "cd /src && makepkg -s --noconfirm --clean"
+
+echo "[entry] Installing freshly built package-manager-*.pkg.tar.*..."
+pacman -U --noconfirm /src/package-manager-*.pkg.tar.*
+
+echo "[entry] Handing off to pkgmgr with args: $*"
+exec pkgmgr "$@"