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>

.dockerignore

@@ -0,0 +1,14 @@
+.git
+.venv
+venv
+__pycache__
+*.pyc
+.pytest_cache
+*.egg-info
+build
+dist
+out
+.claude
+*.sqlite3
+*.sqlite3-wal
+*.sqlite3-shm