feat: add Docker, Docker Compose, and GitHub Actions CI pipeline (#30)

[JuanVP2]

Closes #30

Description

Addresses #30 by introducing comprehensive containerization, local development orchestration, and an automated CI pipeline.

Key Changes

• Production Dockerfile: Implemented an optimized multi-stage build running on Node 24 Alpine under a non-root USER node, explicitly enforcing NODE_ENV=production to respect the security guard for the JWT secret.
• Local Orchestration (docker-compose.yml): Created a development stack linking the NestJS API with a postgres:16-alpine database. Configured an anonymous volume safeguard for node_modules to guarantee Windows/Linux cross-platform compatibility.
• CI Pipeline (.github/workflows/ci.yml): Designed a GitHub Actions workflow triggered on PRs to run linting, building, and test suites (unit & coverage) against an ephemeral Postgres service container, including a security scanning step.
• Documentation: Updated the README.md setup instructions to prioritize the new one-liner Docker workflow.

Verification Results

• Successfully built and verified container multi-stage targets locally.
• Verified localized network communication between NestJS and Postgres via internal compose bindings.
• Swagger documentation verified accessible at http://localhost:3000/api/docs.

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