containers in external network

docker-compose.yaml

# add as a/docker-compose.yaml and b/docker-compose.yaml
# 
# $ docker network create ext-net
# run from 2 terminals (in directory `a`and `b`):
# $ docker-compose up
#
# Should output the same container ID/pseudo-hostname continuously
# 
# tested with docker-compose 1.29.2, 2.0.1, 2.1.0, 2.1.1, 2.2.0
# switching the networks order to [ext-net, my-net] prioritizes name resolution on the common network and makes it fail

services:
  web: 
    image: busybox
    command: sh -c 'while true; do wget -qO - http://db:8000/etc/hostname; sleep 1; done'
    networks: [my-net, ext-net]
  db:
    image: busybox
    command: httpd -f -p 8000 -h /
    ports: [8000]
    networks: [my-net, ext-net]
networks:
  my-net:
  ext-net:
    external: true