.env

POSTGRES_USER=postgres
POSTGRES_PASSWORD=postgres

database.yml

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch('RAILS_MAX_THREADS') { 5 } %>
  host: db
  username: <%= ENV['POSTGRES_USER'] %>
  password: <%= ENV['POSTGRES_PASSWORD'] %>

docker-compose.yml

version: '3.8'

services:
  api:
    restart: always
    networks:
      - backend
    build: .
    env_file: .env
    depends_on:
      db:
        condition: service_healthy
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0' -e development"
    volumes:
      - .:/myapp
      - gem_cache:/usr/local/bundle/gems
    ports:
      - 3000:3000
    deploy:
      replicas: 1

  db:
    restart: always
    networks:
      - backend
    image: postgres:12.4-alpine
    env_file: .env
    healthcheck:
      test: pg_isready -U postgres -h 127.0.0.1
      interval: 5s
    volumes:
      - pg_data:/var/lib/postgresql/data
    deploy:
      replicas: 1
      
 quanto:
    restart: always
    networks:
      - backend
     image: quantocommons/remote-signer
     deploy:
       replicas: 1

volumes:
  pg_data:
  gem_cache:

networks:
  backend:

Dockerfile

FROM ruby:2.7.1-alpine as builder

RUN apk add --update --no-cache \
    build-base \
    postgresql-dev \
    git \
    nodejs \
    yarn \
    tzdata

WORKDIR /myapp

COPY Gemfile Gemfile.lock /myapp/

RUN bundle install -j4 --retry 3 && \
    # Remove unneeded files (cached *.gem, *.o, *.c)
    rm -rf /usr/local/bundle/bundler/gems/*/.git \
    rm -rf /usr/local/bundle/cache/*.gem && \
    find /usr/local/bundle/gems/ -name "*.c" -delete && \
    find /usr/local/bundle/gems/ -name "*.o" -delete

COPY . /myapp

FROM ruby:2.7.1-alpine

RUN apk add --update --no-cache \
    postgresql-client \
    bash \
    tzdata

COPY entrypoint.sh /usr/bin/

RUN chmod +x /usr/bin/entrypoint.sh

COPY --from=builder /usr/local/bundle/ /usr/local/bundle/

COPY --from=builder /myapp /myapp

WORKDIR /myapp

ENTRYPOINT ["entrypoint.sh"]

EXPOSE 3000

CMD ["rails", "server", "-b", "0.0.0.0"]