Simplify Environment Variables

**Description:**
We have simplified the management of environment variables by including the OPENAI_API_KEY in the Docker Compose configuration. This key is now referenced in the .env file and the src/main/resources/application.properties file. Developers only need to create a .env file with their specific API key, eliminating the need to access multiple files.

**Acceptance Criteria:**
Environment Variable Configuration:

OPENAI_API_KEY is included in the Docker Compose file and referenced from the .env file.
application.properties is updated to use the OPENAI_API_KEY environment variable.
Documentation:

Update README.md to include instructions for creating the .env file with the OPENAI_API_KEY.

Start with the Docker Compose configuration, .env usage, src/main/resources/application.properties, and README.md named in the acceptance criteria. Verify that OPENAI_API_KEY is passed from .env into the application properties and that the README explains creating the .env file; done means all three configuration and documentation criteria are satisfied.

Written by the indexing model from the issue text.