The container has a hostname, like mycontainer.lxd, and is accessible from within any container (but not the host, but default).
The IP address you are looking for, is the IP address of the gateway. It does not get a hostname by default and I believe that if there is need, it should be possible to configure LXD's
dnsmasq to produce something.
In most cases, you would rather not get the containers to contact a service in the host, but put all services in containers. Do you have a use-case that would require the containers to contact the host?