Add SECRET_KEY to docker_purldb.env instructions

**Description**
I am a new contributor setting up PurlDB locally using Docker on Windows (WSL).
When running `docker compose up`, the container `purldb-web-1` crashed with the error:
`django.core.exceptions.ImproperlyConfigured: Set the SECRET_KEY environment variable`

**Current Behavior**
The `docker_purldb.env` file (or the instructions) does not include a default `SECRET_KEY`, causing the initial setup to fail for new users.

**Proposed Solution**
I suggest updating the documentation or the `docker_purldb.env` template to include a default key or instructions on how to generate one.

**Environment**
- OS: Windows 11 (WSL2)
- Docker Desktop