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

.dockerignore

@@ -24,3 +24,7 @@ venv/
 .idea/
 .DS_Store
 Thumbs.db
+
+# Arch pkg artifacts
+*.pkg.tar.*
+*.log