Add ln-gateway container build

[dpc]

I intend to use it eventually with podman to deploy ln-gateway as simple bash wrapper:

#!/usr/bin/env bash

exec podman run $MOUNT_OPTS_ETC -q -i docker.io/fedimint/ln-gateway "$@"

This behaves very much like we were running ln_gateway process itself: stdio, stdout, stderr, etc. Though I'm hitting containers/podman#14707 which makes ctrl+c not work.

Re: #1436