Replace hardcoded 'raspberrypi' with system hostname in 511 response

If the system `hostname` is changed from the default `raspberrypi` nginx's 511 redirect will still go to `http://raspberrypi.local:3000` which is wrong.

This filters the response body and replaced `raspberrypi` with the hostname. A bit hacky but I didn't want to go down the route of needing a dynamic application to serve this response.