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

Dockerfile

@@ -13,20 +13,21 @@ ENV NIX_CONFIG="experimental-features = nix-command flakes"
 
 # 2) Unprivileged user for building Arch packages
 RUN useradd -m builder
+
+# 3) Build-Stage (optional): einmal aus /build bauen, wenn du magst
 WORKDIR /build
-
-# 3) Only PKGBUILD rein, um dein Wrapper-Paket zu bauen
 COPY . .
-
 RUN chown -R builder:builder /build \
-    && su builder -c "makepkg -s --noconfirm --clean" \
+    && su builder -c "cd /build && rm -f package-manager-*.pkg.tar.* && makepkg -sf --noconfirm --clean" \
     && pacman -U --noconfirm package-manager-*.pkg.tar.* \
     && rm -rf /build
 
-# 4) Projekt-Quellen für Tests in den Container kopieren
+# 4) Runtime-Workingdir für das gemountete Repo
 WORKDIR /src
-COPY . .
 
-# pkgmgr (Arch-Package) ist installiert und ruft nix run auf.
-ENTRYPOINT ["pkgmgr"]
+# 5) Entry-Script für „always build from /src“
+COPY scripts/docker-entry-dev.sh /usr/local/bin/docker-entry-dev.sh
+RUN chmod +x /usr/local/bin/docker-entry-dev.sh
+
+ENTRYPOINT ["/usr/local/bin/docker-entry-dev.sh"]
 CMD ["--help"]