Fix: Add missing SECRET_KEY to docker env to prevent crash

[Kiran95021]

Problem

When setting up the project locally using Docker, the purldb-web-1 container crashes with:
django.core.exceptions.ImproperlyConfigured: Set the SECRET_KEY environment variable

This happens because the provided docker_purldb.env file is missing a default SECRET_KEY.

Solution

Added a default insecure SECRET_KEY to docker_purldb.env. This allows docker compose up to run successfully for new contributors without manual configuration.

Checklist

• Verified that the container starts successfully with this change.