Add Docker as the primary way to build and run gausse

Dockerfile (non-root user, UID 1000 to match typical host users so bind
mounts don't end up root-owned) + docker-compose.yml with a ./results
volume for the SQLite database and reports that Stage 5/6 will write.
Verified by actually building the image and running the full test suite
and CLI inside the container, not just writing the files and assuming
they work.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -10,5 +10,7 @@ dist/
 *.sqlite3-wal
 *.sqlite3-shm
 out/
+results/*
+!results/.gitkeep
 .DS_Store
 .claude/settings.local.json