Ruby on Rails 6 + Docker = Webpacker::Manifest::MissingEntryError?

Question

Trying to run my Rails app as a Docker container but got the following error when opening a page:

Webpacker can't find application in /app/public/packs/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
   unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}

When I run the app using simply rails s it works fine, and public/packs folder is created. In logs there is also

/usr/local/bundle/gems/webpacker-4.2.0/lib/webpacker/runner.rb:13:in ``': No such file or directory - yarn (Errno::ENOENT)

error, although I checked container yarn -v shows that yarn is installed.

Dockerfile

FROM ruby:2.6.3
RUN apt-get update -qq \
    && curl -sL https://deb.nodesource.com/setup_12.x  | bash - \
    && apt-get install -y nodejs \
    && curl -o- -L https://yarnpkg.com/install.sh | bash
RUN mkdir /app
WORKDIR /app
COPY Gemfile /app/Gemfile
COPY Gemfile.lock /app/Gemfile.lock
RUN bundle install
COPY . /app

# 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"]

docker-compose.yml

version: '3'
services:
  webpacker:
    build: .
    env_file:
      - '.env.docker'
    command: ./bin/webpack-dev-server
    volumes:
      - .:/app
    ports:
      - '3035:3035'
  web:
    build: .
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    volumes:
      - .:/app
    ports:
      - "3000:3000"

.env.docker

NODE_ENV=development
RAILS_ENV=development
WEBPACKER_DEV_SERVER_HOST=0.0.0.0

Answer 1

Follow the below steps.

i) docker-compose run #{rails_app_name} yarn

then again build your project with

ii) docker-compose build #{rails_app_name}

iii) docker-compose up

ref: [1]: https://github.com/rails/webpacker/issues/1568