docker-compose.yml

version: '3'
services:
  postgres:
    image: postgres
    volumes:
      - postgres:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: rails
      POSTGRES_USER: rails
    ports:
      - '5434:5432'

  redis-development:
    image: 'redis:4.0-alpine'
    command: redis-server --requirepass rails
    volumes:
      - 'redis-development:/data'

  sidekiq:
    links:
      - "postgres:db.local"
      - redis-development
    build: .
    command: sidekiq -C config/sidekiq.yml.erb
    volumes:
      - .:/app
    env_file:
      - ./docker_env/.env

  # cable:
  #   depends_on:
  #     - redis
  #   build: .
  #   command: unicorn -p 28080 cable/config.ru
  #   ports:
  #     - '28080:28080'
  #   volumes:
  #     - .:/app
  #   env_file:
  #     - ./docker_env/.env
  #     - ./docker_env/.cable.env

  web:
    build: . 
    links:
      - "postgres:db.local"
      - redis-development
    volumes:
      - .:/app
    ports:
      - '8999:8999'
    env_file:
      - ./docker_env/.env

volumes:
  postgres:
  redis-development:

Dockerfile

ROM ruby:2.6.1
LABEL maintainer="Philip Davies <me@pdavies.io>"

# RUN printenv
# Install dependencies:
# - build-essential: To ensure certain gems can be compiled
# - nodejs: Compile assets
# - libpq-dev: Communicate with postgres through the postgres gem
# because node is shit
RUN curl -sL https://deb.nodesource.com/setup_10.x | bash - && apt-get install -y nodejs
RUN apt-get update  && apt-get install -qq -y build-essential libpq-dev --fix-missing --no-install-recommends
# Unknow reason this wouldnt install psql client inless it was in its own line :(
RUN apt-get install -y postgresql-client-9.6 --fix-missing --no-install-recommends
RUN apt-get install -y nano --fix-missing --no-install-recommends
RUN apt-get install -y graphviz  --fix-missing --no-install-recommends

# Set an environment variable to store where the app is installed to inside
# of the Docker image.
ENV INSTALL_PATH /app
RUN mkdir -p $INSTALL_PATH

# This sets the context of where commands will be ran in and is documented
# on Docker's website extensively.
WORKDIR $INSTALL_PATH

# Ensure gems are cached and only get updated when they change. This will
# drastically increase build times when your gems do not change.
# RUN gem update --system
RUN gem install bundler
COPY Gemfile Gemfile.lock ./
RUN bundle install --binstubs

# Copy in the application code from your work station at the current directory
# over to the working directory.
COPY . .

# Provide dummy data to Rails so it can pre-compile assets.
#RUN bundle exec rake RAILS_ENV=production DATABASE_URL=postgresql://user:pass@127.0.0.1/dbname SECRET_TOKEN=pickasecuretoken assets:precompile

# Expose a volume so that nginx will be able to read in assets in production.
VOLUME ["$INSTALL_PATH/public"]

# The default command that gets ran will be to start the Unicorn server.
CMD ["/bin/bash", "./startup.sh"]

startup.sh

#!/bin/bash

set -e


export DB="$DB_DATABASE"_"$RAILS_ENV"
echo $DB


if [ "$SKIP_PSQL_AVAILABLE" = "true" ]
then
  echo "SKIPPING CHECK OF DATABASE BEING ALIVE"
else
  echo "Checking if Postgres is alive:"

  until PGPASSWORD="$DB_PASS" psql -h "$DB_HOST" -U "$DB_USER" -d "$DB" -c '\q'; do
    >&2 echo "Postgres is unavailable - sleeping"
    sleep 5
  done
  echo "Postgres is alive!"
fi

if [ "$SKIP_DB_SETUP" = "true" ]
then
  echo "SKIPPING SETUP DATABASE"
else
  echo "Checking if Database exists:"
  until PGPASSWORD="$DB_PASS" psql -h "$DB_HOST" -U "$DB_USER" -lqt | cut -d \| -f 1 | grep -qw "$DB"; do
    echo "Database doesn't exist, will run rake db:setup!"
    rake db:setup
    rake db:migrate
  done
  echo "Database has now been setup"
fi


if [ "$SKIP_DB_RESET" = "true" ]
then
  echo "SKIPPING RESETING DATABASE"
else
  echo "db reset"
  rake db:migrate
  rake db:reset
  echo "Database has now been reset"
fi

if [ "$SKIP_DB_SEED" = "true" ]
then
  echo "SKIPPING SEEDING DATABASE"
else
  echo "db seed"
  rake db:seed
  echo "Database has now been seeded"
fi


if [ "$SKIP_DB_DEPLOY" = "true" ]
then
  echo "SKIPPING DATABASE MIGRATIONS"
else
  echo "Deploying Database migrations"
  rake db:migrate
fi

if [ "$SKIP_ASSET_COMPLIE" = "true" ]
then
  echo "SKIPPING ASSET PRE-COMPILE"
else
  echo "RUNNING ASSET PRE-COMPILE"
  bundle exec rake RAILS_ENV=production DATABASE_URL=postgresql://user:pass@127.0.0.1/dbname SECRET_TOKEN=pickasecuretoken assets:precompile
fi

if [ "$SKIP_APP_START" = "true" ]
then
  echo "NOT STARTING APP"
else
  #bundle exec rake assets:precompile
  
  echo "Now starting unicorn!"
  bundle exec unicorn -c config/unicorn.rb
fi

the basics of getting Ruby in Rails Site running in docker. the startup script is a bit of a mess and could be cleaned up and made to be more simple.