Rails 5 & Yarn configuration

I have just added circle to my app and I am using the sample config file laid out in the Rails docs - with a small change to get bundler to work properly:

    version: 2 # use CircleCI 2.0
jobs: # a collection of steps
  build: # runs not using Workflows must have a `build` job as entry point
    parallelism: 1 # run three instances of this job in parallel
    docker: # run the steps with Docker
      - image: circleci/ruby:2.4.2-jessie-node # ...with this image as the primary container; this is where all `steps` will run
        environment: # environment variables for primary container
          BUNDLE_JOBS: 3
          BUNDLE_RETRY: 3
          BUNDLE_PATH: vendor/bundle
          PGUSER: circleci-demo-ruby
          RAILS_ENV: test
      - image: circleci/postgres:9.5-alpine # database image
        environment: # environment variables for database
          POSTGRES_USER: circleci-demo-ruby
          POSTGRES_DB: rails_blog
    steps: # a collection of executable commands
      - checkout # special step to check out source code to working directory

      # Which version of bundler?
      - run:
          name: Which bundler?
          command: bundle -v

      # Restore bundle cache
      # Read about caching dependencies: https://circleci.com/docs/2.0/caching/
      - restore_cache:
            - rails-demo-bundle-v2-{{ checksum "Gemfile.lock" }}
            - rails-demo-bundle-v2-

      - run: # Install Ruby dependencies
          name: Bundle Install
          command: gem install bundler

      # Store bundle cache for Ruby dependencies
      - save_cache:
          key: rails-demo-bundle-v2-{{ checksum "Gemfile.lock" }}
            - vendor/bundle

      # Only necessary if app uses webpacker or yarn in some other way
      - restore_cache:
            - yarn-packages-{{ checksum "yarn.lock" }}

      - run:
          name: Yarn Install
          command: yarn install

      # Store yarn / webpacker cache
      - save_cache:
          key: rails-demo-yarn-{{ checksum "yarn.lock" }}
            - ~/.cache/yarn

      - run:
          name: Wait for DB
          command: dockerize -wait tcp://localhost:5432 -timeout 1m

      - run:
          name: Database setup
          command: bin/rails db:schema:load --trace

      - run:
          name: Run rspec in parallel
          command: |
            bundle exec rspec --profile 10 \
                              --format RspecJunitFormatter \
                              --out test_results/rspec.xml \
                              --format progress \
                              $(circleci tests glob "spec/**/*_spec.rb" | circleci tests split --split-by=timings)

      # Save test results for timing analysis
      - store_test_results: # Upload test results for display in Test Summary: https://circleci.com/docs/2.0/collect-test-data/
          path: test_results

I am getting the error:

#!/bin/bash -eo pipefail yarn install

    yarn install v1.0.2 [1/5] Validating package.json... error @: The engine "node" is incompatible with this module. Expected version "12.16.2". error @: The engine "yarn" is incompatible with this module. Expected version "1.22.4". error Found incompatible module info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command. Exited with code exit status 1

    CircleCI received exit code 1

In my package.json I explicitly set the node and yarn versions. Whatever I try, it still attempts to install a different yarn version (1.0.2). How do I specify the version that I need? Thanks