feat: add docker containerization for keeper service (#79)

[Abdullahi-Code9]

Closes #585

📝 Description

This PR introduces full Docker containerization for the PayFlow keeper service. It provides a multi-stage Dockerfile, a production-ready docker-compose.yml, environment configuration templates, isolated build configurations, and comprehensive operational documentation in scripts/README.md.

🚀 Changes Made

1. Multi-Stage Dockerfile (scripts/Dockerfile)

• Built with a 2-stage setup using node:20-alpine:
  • Stage 1 (Builder): Compiles TypeScript using tsconfig.build.json.
  • Stage 2 (Runtime): Minimal slim image with production dependencies only.
• Executed securely under non-root USER node.
• Configured container health monitoring via native wget health checks.

2. Docker Compose & Environment setup (docker-compose.yml, .env.example)

• Added docker-compose.yml configured with restart: unless-stopped, 60-second graceful stop timeouts, rotating JSON log driver, and resource caps.
• Provided .env.example documenting all 7 keeper environment variables.

3. Core Keeper Logic & Documentation (scripts/keeper.ts, scripts/README.md)

• Added scripts/keeper.ts supporting paginated batch_charge(offset, limit) execution with structured JSON logging for aggregator compatibility and graceful SIGINT/SIGTERM shutdown handling.
• Authored documentation covering local execution, Docker deployment, environment variables, and health checks.

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🧪 Verification & Testing

Type Safety Verification

Verified strict TypeScript compilation:

npx tsc --noEmit

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