How to get long hostname of a docker container inside a container? - docker-compose

Question

Our application is behind traefik reverse proxy. We manage many subdomains and we use the watch-file ability of traefik to dynamically setup new subdomains to proxyfy.
So our application generate a traefik .yaml dynamic config file.
The same traefik will manage many instances of the same application.
For that purprose we need to indicate to traefik how to reach our application inside it own network.
We know we can use the simple hostname, the one which is the container name.
But this only work inside the default docker-compose network of the app instance and not the external network shared with traefik.
This one need the long hostname version so we are sure it reach the right application instance.
(<compose_name>_<container_name>_1 or depending docker-compose version (<compose_name>_<container_name>_1_<hash>)
Do you know a way to get the long version of the hostname of a docker-compose container inside another container of the same docker-compose default network ?
For better context, here a simple docker-compose.yaml file
version: "3"
services:
app:
image: app_image
networks:
- app_network
restart: unless-stopped
nginx:
image: nginx
links:
- app
networks:
- app_network
- traefik_traefik
restart: unless-stopped
networks:
traefik_traefik:
external: true
app_network:
driver: bridge
We want, from inside the app container, to get the nginx long hostname version, so we can use it to generate the dynamic configuration for traefik.
Thanks for your help.

Answer 1

We thought to have found a solution. Querying for the FQDN of the short-named hostname give the needed long hostname.
dig +short -x `dig +short nginx`
Return composename_app_1.composename.app_network.
In our python app we can get the same result with
import socket
socket.getfqdn('nginx')