I came up with a workflow that uses s3 to cache between branches

###################################################################
# Sample Github workflow to cache deps/ and _build/ in S3
# between branches
###################################################################
# S3 credentials needed (define these as Github secrets)
# - AWS_ACCESS_KEY_ID
# - AWS_SECRET_ACCESS_KEY
# - BUILD_BUCKET
# 
# If you have private Git repos, you need to add a private key to the repo
# - HEADLESS_PRIV
name: Test

on:
  pull_request:
    branches:
      - develop
    paths-ignore:
      - 'docs/**'
      - '*.md'

jobs:
  test:

    name: Tests

    runs-on: ubuntu-18.04

    env:
      MIX_ENV: test

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

    steps:
      - uses: actions/checkout@v2
      - uses: erlef/setup-elixir@v1
        with:
          otp-version: '23.1.2'
          elixir-version: '1.11.2'

      - name: Configure SSH for private Repos
        uses: webfactory/ssh-agent@v0.4.1
        with:
          ssh-private-key: ${{ secrets.HEADLESS_PRIV }}

      - name: Restore Dependencies from S3 (if present)
        id: restore-deps-cache
        run: '(aws s3 ls s3://${{ secrets.BUILD_BUCKET }}/some-dir/deps.tar && aws s3 cp s3://${{ secrets.BUILD_BUCKET }}/some-dir/deps.tar deps.tar) || echo deps.tar not found on S3, skipping step'
        env:
          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          AWS_REGION: 'us-east-1'

      - name: Restore Build from S3 (if present)
        id: restore-build-cache
        run: '(aws s3 ls s3://${{ secrets.BUILD_BUCKET }}/some-dir/_build.tar && aws s3 cp s3://${{ secrets.BUILD_BUCKET }}/some-dir/_build.tar _build.tar) || echo _build.tar not found on S3, skipping step'
        env:
          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          AWS_REGION: 'us-east-1'

      - name: 'Untar deps.tar (if present)'
        run: '(test -f deps.tar && tar -xvf deps.tar) || echo deps.tar not found, skipping step'

      - name: 'Untar _build.tar (if present)'
        run: '(test -f _build.tar && tar -xvf _build.tar) || echo _build.tar not found, skipping step'

      - name: Get dependencies
        run: |
          mix local.hex --force
          mix local.rebar
          mix deps.get

      - name: Compile
        run: mix compile

      - name: Create database
        run: mix ecto.create

      - name: Migrate database
        run: mix ecto.migrate

      - name: Test
        run: mix test

      - name: 'Tar deps'
        run: sudo tar -cvf deps.tar deps

      - name: 'Tar _build'
        run: sudo tar -cvf _build.tar _build

      - name: Cache Dependencies to S3
        id: deps-cache
        run: '(test -f deps.tar && aws s3 cp deps.tar s3://${{ secrets.BUILD_BUCKET }}/some-dir/deps.tar) || echo deps.tar not found locally, skipping step'
        env:
          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          AWS_REGION: 'us-east-1'

      - name: Cache Build to S3
        id: build-cache
        run: '(test -f _build.tar && aws s3 cp _build.tar s3://${{ secrets.BUILD_BUCKET }}/some-dir/_build.tar) || echo _build.tar not found locally, skipping step'
        env:
          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          AWS_REGION: 'us-east-1'

Thank you! This is helpful. I also looked to the second workflow in your link. I think my workflow is ok, but it is still slow. With no cache, it takes 3 minutes. With cache it takes about 1 minutes if there are only changes in app. If there are other changes, it takes about 2 minutes. I do not see anything clear that I maybe could do better. Thank you for your input!