Update start.sh

[DawoodTouseef]

Key Changes:
Using gunicorn Instead of uvicorn:

Replace uvicorn with gunicorn while using the UvicornWorker to maintain compatibility with ASGI applications.
Command Format for gunicorn:

The uvicorn main:app is replaced by gunicorn main:app --worker-class uvicorn.workers.UvicornWorker.
The --host "$HOST" and --port "$PORT" options for uvicorn are replaced with --bind "$HOST:$PORT" in gunicorn.
Startup and Shutdown Behavior:

The logic for starting and stopping the web server during admin user creation remains the same, just with gunicorn commands.
With this script multiple request can be accepted it a same time.

[sir3mat]

Is it possible to have workers params as in the pipeline server? See this issue pipelines_ref