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>

docker-compose.yml

@@ -0,0 +1,7 @@
+services:
+  gausse:
+    build: .
+    volumes:
+      - ./results:/app/results
+    entrypoint: ["gausse"]
+    command: ["--help"]