create_rails_inside_docker.md

1. Create simple Gemfile

source 'https://rubygems.org'
gem 'rails', '~> 6.1`

2. Create empty Gemfile.lock

touch Gemfile.lock

3. Create Dockerfile

FROM ruby:2.6.8

RUN apt-get update -qq && apt-get install -y build-essential libpq-dev
RUN curl -sL https://deb.nodesource.com/setup_14.x | bash - && \
    apt-get install -y -qq nodejs && npm install --global yarn

RUN mkdir /app
WORKDIR /app

ADD Gemfile /app/Gemfile
ADD Gemfile.lock /app/Gemfile.lock

RUN bundle install

ADD . /app

4. Create docker-compose.yml

version: '3.3'

services:
  db:
    image: postgres:12-alpine
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: app_database
      POSTGRES_HOST_AUTH_METHOD: trust
    ports:
      - 5432:5433
  app:
    build: .
    command: bundle exec rails s -p 3000 -b 0.0.0.0
    environment:
      POSTGRES_HOST: db
    volumes:
      - '.:/app'
    ports:
      - 3001:3000
    depends_on:
      - db
    links:
      - db

5. Inside folder run command

docker-compose run app rails new . --force --database=postgresql --skip-bundle

6. After creating app edit config/database.yml

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: <%= ENV['PG_DB'] %>
  username: <%= ENV['PG_USER'] %>
  password: <%= ENV['PG_PASSWORD'] %>
  host: <%= ENV['PG_HOST'] %>

test:
  <<: *default
  database: app_test

production:
  <<: *default
  database: app_production
  username: app
  password: <%= ENV['APP_DATABASE_PASSWORD'] %>

7. Add environment variables to docker-compose.yml in app: part

environment:
  PG_DB: app_database
  PG_USER: postgres
  PG_PASSWORD: postgres
  PG_HOST: db

8. Run docker-compose build. This install all gems. NOTE: If there are some problems with rights need to run command sudo chown -R $USER:$USER /path/to/app

9. Run migrations, generators, etc. For example run migrations.

docker-compose run --rm app rake db:migrate

10. Install webpacker

docker-compose run --rm app rails webpacker:install

11. Up application.

docker-compose up

and open page http://localhost:3001