Set up Dockerfiles for API Gateway and Microservices

Create Dockerfiles for all microservices, ensuring they run inside containers with the appropriate dependencies. Each microservice should be containerised properly, using the single venv approach for dependencies.

Tasks:

• Set up Dockerfile for API Gateway
• Set up Dockerfile for User Service
• Set up Dockerfile for Transaction Service
• Set up Dockerfile for Budget Service
• Set up Dockerfile for Analytics Service
• Set up Dockerfile for Notification Service
• Ensure compatibility with docker-compose.yml

Acceptance Criteria:

• Each Dockerfile should install dependencies from the project-wide venv (requirements.txt in the root folder).
• Containers should expose correct ports for communication.
• All microservices should start successfully using docker-compose up --build.