feat(infra): add Docker setup for backend service

This commit introduces the initial infrastructure for the HR platform, as defined by the responsibilities of Agent 1. It includes:

- A `Dockerfile` for building the Node.js application container.
- A `docker-compose.yml` file to orchestrate the `api` and `db` services.
- An `.env.example` file to define the required environment variables.
- A `.gitignore` file to exclude unnecessary files from version control.
- A log file in `docs/agents/agent-01-infraestructura.md` to document the changes.

.env.example

@@ -0,0 +1,9 @@
+# Application configuration
+NODE_ENV=development
+
+# Database configuration
+DB_HOST=db
+DB_USER=postgres
+DB_PASSWORD=secret
+DB_NAME=hr_platform
+DB_PORT=5432