elixir.yml

name: Elixir CI

on:
  push:
    branches:
      - master

jobs:
  test:
    runs-on: ubuntu-latest

    services:
      db:
        image: postgres:11
        ports: ["5432:5432"]
        options: >-
          --health-cmd pg_isready
          --health-interval 10s
          --health-timeout 5s
          --health-retries 5

    steps:
      - uses: actions/checkout@v1.0.0
      - uses: actions/setup-elixir@v1.0.0
        with:
          otp-version: 22.0
          elixir-version: 1.9.4
      - uses: actions/setup-node@v1
        with:
          node-version: 12
      - name: Install chromedriver
        run: |
          wget https://chromedriver.storage.googleapis.com/78.0.3904.105/chromedriver_linux64.zip
          unzip chromedriver_linux64.zip
          sudo mv chromedriver /usr/bin/chromedriver
          sudo chown root:root /usr/bin/chromedriver
          sudo chmod +x /usr/bin/chromedriver

      # Cache NPM packages
      - name: Retrieve NPM Packages Cache
        uses: actions/cache@v1
        id: npm-cache
        with:
          path: assets/node_modules
          key: ${{ runner.os }}-npm-${{ hashFiles(format('{0}{1}', github.workspace, '/assets/package-lock.json')) }}

      # Install NPM Packages only if cache was not hit
      - name: Install NPM Packages
        if: steps.npm-cache.outputs.cache-hit != 'true'
        run: npm install --prefix assets

      # Cache Mix Dependencies based on mix.lock
      - name: Retrieve Mix Dependencies Cache
        uses: actions/cache@v1
        id: mix-cache
        with:
          path: deps
          key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}

      # Get Mix Dependencies only if cache was not hit
      - name: Install Mix Dependencies
        if: steps.mix-cache.outputs.cache-hit != 'true'
        run: |
          mix local.rebar --force
          mix local.hex --force
          mix deps.get

      # Compile and deploy assets to priv
      - name: Deploy assets
        run: npm run deploy --prefix ./assets

      # Cache the _build folder
      - name: Retrieve Build Cache
        uses: actions/cache@v1
        id: build-cache
        with:
          path: _build
          key: ${{ runner.os }}-build-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}

      # Compile mix dependencies if the _build folder was not cached
      - name: Compile Mix Dependencies
        if: steps.build-cache.outputs.cache-hit != 'true'
        run: MIX_ENV=test mix deps.compile

      # Run the tests
      - name: Run Tests
        run: mix test

Pretty great setup! Compiling deps first and then the code is a neat trick.
I am using this file as a reference :)
I only changed one thing. I've added elixir and OTP versions to cache keys. I used matrix strategy for that even though there will be only one build in the matrix.

    strategy:
      matrix:
        elixir-version: [1.9.4]
        otp-version: [22.2]

    steps:
    - uses: actions/checkout@v2
    - name: Setup elixir
      uses: actions/setup-elixir@v1
      with:
        elixir-version: ${{ matrix.elixir-version }}
        otp-version: ${{ matrix.otp-version }}

        key: ${{ runner.os }}-${{ matrix.otp-version }}-${{ matrix.elixir-version }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}

Uh! Yes! That's a great idea! I'll steal that from you if you don't mind :P

Of course, I don't mind :) That's why I am sharing it :) And to let you know that I've found your setup with separate step for compiling deps very useful! :)

I also added Dialyxir to my setup with separate step for compiling PLTs and another one for actual check: https://github.com/tomekowal/composable_html/blob/master/.github/workflows/elixir.yml

The dialyzer part is not perfect because it requires repeating priv/plts in elixir.yml and mix.exs but does the job :)

Wow, lots of good thing here, thank you!
Unless you need a specific version of Chrome/Driver, it's installed by default on ubuntu-latest
https://github.com/actions/virtual-environments/blob/master/images/linux/Ubuntu1804-README.md

Wow, lots of good thing here, thank you!
Unless you need a specific version of Chrome/Driver, it's installed by default on ubuntu-latest
https://github.com/actions/virtual-environments/blob/master/images/linux/Ubuntu1804-README.md

Good work. Thanks