How to upgrade yarn in CircleCi 2.0?

Build Environment
1

I’m trying to upgrade to CircleCI 2.0 for a Rails app. I used the automated config updater tool, and the auto-generated tool resulted in failures when the command bin/rake assets:precompile ran, with the error Webpacker requires Yarn >= 0.25.2 and you are using 0.21.3.

To fix it I tried manually updating yarn as one of my build steps, meaning my config file now starts like this:

version: 2
jobs:
  build:
    working_directory: ~/XX/YYY
    parallelism: 1
    shell: /bin/bash --login
    environment:
      CIRCLE_ARTIFACTS: /tmp/circleci-artifacts
      CIRCLE_TEST_REPORTS: /tmp/circleci-test-results
    docker:
    - image: circleci/build-image:ubuntu-14.04-XXL-upstart-1189-5614f37
      command: /sbin/init
    steps:
    - checkout
    - run: mkdir -p $CIRCLE_ARTIFACTS $CIRCLE_TEST_REPORTS
    - run:
        working_directory: ~/XX/YYY
        command: rm -f XX/YYY/.rvmrc; echo 2.3.1 > XX/YYY/.ruby-version; rvm use 2.3.1 --default
    - run:
        working_directory: ~/XX/YYY
        command: nvm install 8.9.4 && nvm alias default 8.9.4
    - run: curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
    - run: echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
    - run: sudo apt-get update && sudo apt-get install yarn && yarn -v

Given that sudo apt-get install yarn installs v1.5.1 (the last line of the output is Setting up yarn (1.5.1-1)), I would have assumed that the output of yarn -v would be v1.5.1. But actually the build fails on this command, with the error:

yarn install v0.21.3
[1/4] Resolving packages...
[2/4] Fetching packages...
error @rails/webpacker@3.0.2: The engine "yarn" is incompatible with this module. 
Expected version ">=0.25.2".
error Found incompatible module

I can’t for the life of me understand why yarn -v just after I installed v1.5.1 is prompting v0.21.3 to be installed, and why webpacker is getting involved at this stage. It looks like the first time I try to invoke yarn, something somewhere says to go fetch 0.21.3 - so how do I bump that number?

Thanks!

2

This build image comes preinstalled with yarn 0.21.3 and node 4.2.6. Installing via apt won’t remove them from your system or your PATH. In order to run a newer version of either, you need to (a) modify your PATH (b) fully-qualify the path to the executable, or (c) use another base image e.g. https://circleci.com/docs/2.0/circleci-images/#ruby