refactor(docker): improve Docker setup and reduce image size

- Add multi-stage build to Dockerfile for smaller production image
- Move docker-compose.yml to project root for easier usage
- Flatten docker/init/setup.sh to docker/setup.sh
- Expand .dockerignore to exclude unnecessary files
- Remove deprecated version field from compose files

docker/docker-compose.test.yml

@@ -1,5 +1,5 @@
 # Docker Compose configuration for running integration tests
-# Usage: docker-compose -f docker/docker-compose.test.yml up -d
+# Usage: docker compose -f docker/docker-compose.test.yml up -d
 services:
   test-db:
     image: postgres:15-alpine