前提

MacにDocker Desktopがインストールされていること。
適宜、プロジェクト用のディレクトリを用意する。

Dockerfile

FROM ruby:2.6.5

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
    && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list

RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs yarn

RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
COPY . /myapp

# Add a script to be executed every time the container starts.
COPY entrypoint.sh /usr/bin/
RUN chmod +x /usr/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]
EXPOSE 3000

# Start the main process.
CMD ["rails", "server", "-b", "0.0.0.0"]

Gemfile

source 'https://rubygems.org'
gem 'rails', '~>6'

Gilefile.lock

touch Gemfile.lock

entrypoint.sh

#!/bin/bash
set -e

# Remove a potentially pre-existing server.pid for Rails.
rm -f /myapp/tmp/pids/server.pid

# Then exec the container's main process (what's set as CMD in the Dockerfile).
exec "$@"

docker-compose.yml

version: '3'
services:
  db:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: password
      MYSQL_DATABASE: root
    ports:
      - "3306:3306"
    volumes:
      - ./docker/db/data:/var/lib/mysql
      - ./docker/db/my.cnf:/etc/mysql/conf.d/my.cnf
  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
      - .:/myapp
    ports:
      - "3000:3000"
    depends_on:
      - db

Railsプロジェクトの作成

# 薄いRailsプロジェクト
docker-compose run web rails new . --force –-database=mysql --skip-action-mailer –-skip-action-cable –-skip-sprockets --skip-test --skip-turbolinks -–skip-bundle

build

docker-compose build

database.ymlの修正

config/database.yml

default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: root
  password: password # docker-compose.yml の MYSQL_ROOT_PASSWORD
  host: db

コンテナ再起動

docker-compose down
docker-compose up -d

Webpacker

docker-compose run web bundle exec rails webpacker:install

DB作成

docker-compose run web bundle exec rails db:create