docker-compose: confirm a dependent container is launched and accepting TCP connections

docker-compose.yml.example

version: '2'

services:
  rspec:
    build:
      context: .
      dockerfile: Dockerfile_rspec_example
    environment:
      APP_HOST: web.app.docker
      HUB_HOST: selenium.hub.docker
    volumes:
      - .:/usr/src/app
    depends_on:
      - web
      - hub

  web:
    image: web-app:latest
    environment:
      RAILS_ENV: test
      VIRTUAL_HOST: web.app.docker

  hub:
    image: selenium/hub:2.52.0
    environment:
      VIRTUAL_HOST: selenium.hub.docker
    ports:
      - 4444:4444

Dockerfile_rspec_example

FROM ruby:latest

USER root

ENV APP_HOME /usr/src/app
RUN mkdir -p $APP_HOME
WORKDIR $APP_HOME

RUN apt-get update && apt-get install -y \
  netcat-openbsd

COPY ./wait $APP_HOME

ENTRYPOINT bash wait

wait

#!/bin/bash

# Any docker container may use this script to ensure a dependent container is
# launched and listening on TCP

# args: dependent containers's host address and port

set -e

host=$HUB_HOST
port=$HUB_PORT

echo -n "waiting for TCP connection to $host:$port..."

while ! nc -w 1 $host $port 2>/dev/null
do
  echo -n .
  sleep 1
done

echo 'ok'