How to share a /usr/local/bundle cache between Ruby build jobs

docker-compose.yml

version: "2"

services:

  dev:
    image: ruby:2.3
    volumes:
      - .:/project
      - ruby2.3-bundle-cache:/usr/local/bundle
    working_dir: /project

volumes:
  ruby2.3-bundle-cache:
    external: true

rake.sh

#! /bin/bash -e
#
# Run rake inside a docker-compose sandbox

cd $(dirname $0)/..

trap "docker-compose down --volumes" 0

docker volume create --name ruby2.3-bundle-cache > /dev/null
docker-compose run --rm dev sh -c 'bundle check > /dev/null || flock $GEM_HOME bundle install'
docker-compose run --rm dev bundle exec rake "$@"

Note the flock $GEM_HOME stuff. We do because Bundler (or RubyGems) gets upset if you have multiple "bundle install" jobs pointed at the same directory.