This docker-compose.yml seems to work OK:

version: '3'
services:
  web:
    image: web-app
    command: bundle exec rackup
    ports:
     - "9292:9292"
    links:
      - redis
  redis:
    image: redis

Command:

docker build -t web-app .; docker-compose up

The Web App is expecting a REDIS_URL config. Web App Dockerfile:

ENV REDIS_URL redis:6379

It seems that the Web App cannot connect to the Redis:

Redis::CannotConnectError - Error connecting to Redis on 127.0.0.1:6379 (Errno::ECONNREFUSED):

So, what is the Redis URL for the Web App?

Do I need to expose the Redis port (6379)?

EDIT:

Thanks to @Andy Shinn, I see the redis url should be redis://6379.
Now I get Errno::EINVAL - Invalid argument - connect(2) for 0.0.24.235:6379. This may have to do with Redis v 3.2.8. I'll try another version and see if it works.

EDIT #2:

3.2.8 works fine with the app on my local machine, so that is not the issue.