Clarify local environment setup steps in deployment/docker/README.md

Description

While setting up a local Chainlink node using the Docker Compose configuration in deployment/docker/, a few critical environment variables are not clearly documented.
New contributors may run into DATABASE_URL and ETH_URL missing errors even after following the provided instructions.

Steps to Reproduce

1. Clone the repository and navigate to deployment/docker/
2. Copy .env.example → .env
3. Run docker-compose up
4. Observe missing variable or connection errors in logs

Expected Behavior

The README should:

• Clearly indicate which variables are mandatory
• Provide example values for a local setup (e.g. Ganache/Hardhat)
• Include a minimal working .env snippet

Actual Behavior

The instructions imply .env.example is self-sufficient, but it isn’t.

Suggested Fix

Add a “Quick Start (Local Development)” section in deployment/docker/README.md showing a verified minimal configuration and linking to official environment variable references.

Environment

• OS: Ubuntu 24.04 / macOS Sonoma
• Docker: 27.x
• Branch: develop