Github actions matrix build for all packages in a monorepo

readme.md

assumes a file structure something like:

├── package.json
└── packages
    ├── foo
    │   ├── package.json
    │   └── ...
    └── bar
        ├── package.json
        └── ...

build.yml

name: build
on:
  push:
    branches:
      - main
jobs:
  matrix:
    runs-on: ubuntu-latest
    outputs:
      packages: ${{ steps.set_packages.outputs.packages }}
    steps:
      - uses: actions/checkout@v2
      - id: set_packages
        run: |
          echo "::set-output name=packages::$(
            find packages -mindepth 1 -maxdepth 1 -exec basename '{}' ';' \
            | jq --compact-output --raw-input -n '[inputs]'
          )"
  build:
    name: Build ${{ matrix.package }}
    runs-on: ubuntu-latest
    needs: matrix
    strategy:
      fail-fast: false
      matrix:
        package: ${{ fromJson(needs.matrix.outputs.packages) }}
    defaults:
      run:
        working-directory: ./packages/${{ matrix.package }}
    steps:
      - uses: actions/checkout@v2
      - name: Set up Node
        uses: actions/setup-node@v2
        with:
          node-version: "16"
      - name: Install dependencies
        run: npm install
      - name: Build
        run: npm run build