Dockerfile returns npm not found on build - docker

Question

I'm really new to Docker and would like to create a container that has all my Ruby on Rails website and its dependencies in it. I read TONS of documentations and how-to's to do it but I keep struggling.
Since I use Rubymine, there is a built-in tool that allows you to Dockerize the current project you're in from the Dockerfile you created in it. However, I keep having the error "npm not found while NodeJS should be installed.
Here is my Dockerfile:
FROM ruby:2.4.3
MAINTAINER Jaeger
RUN apt-get update -qq && apt-get install -y build-essential nodejs
RUN mkdir -p /app
WORKDIR /app
COPY package.json /app
RUN npm i -g yarn && yarn
COPY Gemfile Gemfile.lock ./
RUN gem install bundler && bundle install --jobs 20 --retry 5
RUN yarn
COPY . ./
EXPOSE 3000
CMD ["bundle", "exec", "rails", "server", "-b", "0.0.0.0"]
The ultimate goal should be that, since I use Webpacker and React in it (well it's a dummy project for a test, but the real website has this), I would like to install Yarn, and make Yarn install all the depencies.
I found some other people that had the same problem but got lost trying to understand the Docker layers concept and trying to copy some codes that didn't work either

Answer 1

You need to explicitly install node / npm in your container before running npm install. Add this to your Dockerfile.
RUN apt-get update && apt-get install -y curl
RUN curl -sL https://deb.nodesource.com/setup_8.x | bash -
RUN apt-get update && apt-get install -y nodejs

Answer 2

Another solution (which might be cleaner) would be to first mount / setup the working directory before installing the dependencies:
FROM ruby:2.4.3
MAINTAINER Jaeger
RUN mkdir -p /app
WORKDIR /app
RUN apt-get update -qq && apt-get install -y build-essential nodejs
...