Listen IP address

Hi,

I'm facing a problem where I need the sync-service's HTTP API to listen on a specific IP address on my server rather than simply listening on "0.0.0.0" or "127.0.0.1".

A solution that would work for me is to add an environment variable named `ELECTRIC_IP` that configures the HTTP API to listen on a specific IP address. I see that an environment variable named `ELECTRIC_PORT` already exists, so this new `ELECTRIC_IP` would work similarly.