title: Ruby on Rails 6 + Docker = Webpacker::Manifest::MissingEntryError?
body: <p>Trying to run my Rails app as a Docker container but got the following error when opening a page:</p>
  
  <pre><code>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:
  {
  }
  </code></pre>
  
  <p>When I run the app using simply <code>rails s</code> it works fine, and <strong>public/packs</strong> folder is created. 
  In logs there is also</p>
  
  <pre><code>/usr/local/bundle/gems/webpacker-4.2.0/lib/webpacker/runner.rb:13:in ``': No such file or directory - yarn (Errno::ENOENT)
  </code></pre>
  
  <p>error, although I checked container <strong>yarn -v</strong> shows that yarn is installed.</p>
  
  <p>Dockerfile</p>
  
  <pre><code>FROM ruby:2.6.3
  RUN apt-get update -qq \
      &amp;&amp; curl -sL https://deb.nodesource.com/setup_12.x  | bash - \
      &amp;&amp; apt-get install -y nodejs \
      &amp;&amp; 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"]
  </code></pre>
  
  <p>docker-compose.yml</p>
  
  <pre><code>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 &amp;&amp; bundle exec rails s -p 3000 -b '0.0.0.0'"
      volumes:
        - .:/app
      ports:
        - "3000:3000"
  </code></pre>
  
  <p>.env.docker</p>
  
  <pre><code>NODE_ENV=development
  RAILS_ENV=development
  WEBPACKER_DEV_SERVER_HOST=0.0.0.0
  </code></pre>