[Request] Update air gapped docs to show how to run in production

## Description

The [documentation on hosting your own package repo](https://www.elastic.co/guide/en/fleet/current/air-gapped.html) has instructions on running the container in the foreground. It would be more practical in a production environment to have the container run detached and potentially automatically restart.

something like:
`docker run -d --restart=unless-stopped -it -p 8080:8080 docker.elastic.co/package-registry/distribution:8.0.0`