Reorganize Docker-related files into docker/ directory

Moved Docker infrastructure files to dedicated docker/ folder:
- config/nginx/default.conf → docker/config/nginx/default.conf
- init.sql → docker/init.sql
- start.sh → docker/start.sh (currently unused)

Updated:
- docker-compose.yml: Adjusted volume paths
- README.md: Updated project structure documentation

Benefits:
- Clear separation between infrastructure (docker/) and application (src/)
- Better project organization
- Easier to understand for new developers

Docker Compose and Dockerfile remain in root for convenience.
All services tested and working correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -15,7 +15,7 @@ services:
       - ./composer.lock:/var/www/composer.lock
       - ./phpstan.neon:/var/www/phpstan.neon
       - ./phpcs.xml:/var/www/phpcs.xml
-      - ./config/nginx/default.conf:/etc/nginx/conf.d/default.conf
+      - ./docker/config/nginx/default.conf:/etc/nginx/conf.d/default.conf
     depends_on:
       - mariadb
     networks:
@@ -34,7 +34,7 @@ services:
       - "3307:3306"
     volumes:
       - mariadb_data:/var/lib/mysql
-      - ./init.sql:/docker-entrypoint-initdb.d/init.sql
+      - ./docker/init.sql:/docker-entrypoint-initdb.d/init.sql
     networks:
       - app-network