Good starter Dockerfile + docker-compose.yml for Rails 6.

README.md

1. docker-compose build app
2. docker-compose up

docker-compose run --rm app bundle # Gemfile change

docker compose exec app rake db:create

Thanks to @jfahrer for the Railsconf talk that taught me this

docker-compose.yml

  version: '3.7'

  services:
    app:
      image: your_user_name_here/appname:v1
      build:
        context: .
      command: ["spring", "server"]
      environment:
        - POSTGRES_HOST=pg
        - POSTGRES_USER=postgres
        - POSTGRES_PASSWORD=secret
        - RAILS_ENV
      volumes:
        - ./:/usr/src/app
        - gems:/usr/local/bundle
        - node-modules:/usr/src/app/node_modules
        - ${HOME}/.irbrc:/root/.irbrc
        - ${HOME}/.irb-history:/root/.irb-history

    web:
      image: your_user_name_here/appname:v1
      command: ["rails", "server", "-b", "0.0.0.0"]
      environment:
        - POSTGRES_HOST=pg
        - POSTGRES_USER=postgres
        - POSTGRES_PASSWORD=secret
        - RAILS_ENV
      volumes:
        - ./:/usr/src/app
        - gems:/usr/local/bundle
        - node-modules:/usr/src/app/node_modules
      ports:
        - 3000:3000

    pg:
      image: postgres:11.2-alpine
      volumes:
        - pg-data:/var/lib/postgresql/data
      environment:
        - POSTGRES_USER=postgres
        - POSTGRES_PASSWORD=secret

    webpacker:
      image: your_user_name_here/appname:v1
      command: ./bin/webpack-dev-server
      volumes:
        - ./:/usr/src/app
        - gems:/usr/local/bundle
        - node-modules:/usr/src/app/node_modules
      ports:
        - 3035:3035


  volumes:
    gems:
    pg-data:
    node-modules:

Dockerfile

FROM ruby:2.6.5-alpine

RUN apk add --update --no-cache bash build-base nodejs sqlite-dev tzdata postgresql-dev yarn

RUN gem install bundler:2.1.4

WORKDIR /usr/src/app

COPY package.json yarn.lock ./
RUN yarn install --check-files

COPY Gemfile* ./
RUN bundle install

COPY . .

ENV PATH=./bin:$PATH
CMD ["rails", "console"]

webpacker.yml

# config/webpacker.yml

# Note: You must restart bin/webpack-dev-server for changes to take effect

default: &default
  source_path: app/javascript
  source_entry_path: packs
  public_root_path: public
  public_output_path: packs
  cache_path: tmp/cache/webpacker
  check_yarn_integrity: false
  webpack_compile_output: false

  # Additional paths webpack should lookup modules
  # ['app/assets', 'engine/foo/app/assets']
  resolved_paths: []

  # Reload manifest.json on all requests so we reload latest compiled packs
  cache_manifest: false

  # Extract and emit a css file
  extract_css: false

  static_assets_extensions:
    - .jpg
    - .jpeg
    - .png
    - .gif
    - .tiff
    - .ico
    - .svg
    - .eot
    - .otf
    - .ttf
    - .woff
    - .woff2

  extensions:
    - .mjs
    - .js
    - .sass
    - .scss
    - .css
    - .module.sass
    - .module.scss
    - .module.css
    - .png
    - .svg
    - .gif
    - .jpeg
    - .jpg

development:
  <<: *default
  compile: true

  # Verifies that versions and hashed value of the package contents in the project's package.json
  # check_yarn_integrity: true

  # Reference: https://webpack.js.org/configuration/dev-server/
  dev_server:
    https: false
    host: webpacker
    port: 3035
    public: localhost:3035
    hmr: true
    # Inline should be set to true if using HMR
    inline: true
    overlay: true
    compress: true
    disable_host_check: true
    use_local_ip: false
    quiet: false
    headers:
      'Access-Control-Allow-Origin': '*'
    watch_options:
      ignored: '**/node_modules/**'


test:
  <<: *default
  compile: true

  # Compile test packs to a separate directory
  public_output_path: packs-test

production:
  <<: *default

  # Production depends on precompilation of packs prior to booting for performance.
  compile: false

  # Extract and emit a css file
  extract_css: true

  # Cache manifest.json for performance
  cache_manifest: true

I think having RUN bundle install is somewhat incompatible with having the gems:/usr/local/bundle volume in docker-compose. Basically, on running the container, it's bundle folder will immediately be replaces by the contents of /gems. Of course, you still want the gems in the image for pushing, but in this case you might as well just copy them in COPY /gems /usr/local/bundle

The purpose of the volume is to persist the gems, so you only have to run bundle install in case the Gemfile changes (same with the node_modules).

Dockerfile failed to build on line #9.
How should I provide that from host system.

COPY failed: file not found in build context or excluded by .dockerignore: stat package.json: file does not exist