fix: update environment variable handling in Docker Compose files and improve contributing documentation

nest-backend/docker-compose.yml

@@ -18,7 +18,9 @@ services:
     build: .
     restart: unless-stopped
     ports:
-      - "3000:3000"
+      - "${API_HOST_PORT:-3001}:3000"
+    env_file:
+      - ../.env
     environment:
       DATABASE_URL: postgresql://nest_user:nest_password@db:5432/nest_db
       JWT_SECRET: ${JWT_SECRET:-change_me_in_production}