Deploy a Phoenix app with GitHub Actions on fly.io with Docker layer caching

deploy-production.yml

# Based on https://evilmartians.com/chronicles/build-images-on-github-actions-with-docker-layer-caching
on:
  push:
    branches:
      - main

jobs:
  deploy_production:
    name: Deploy production
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2

      - name: Prepare
        id: prep
        run: |
          TAG=$(echo $GITHUB_SHA | head -c7)
          IMAGE="my_app"
          echo ::set-output name=tagged_image::${IMAGE}:${TAG}
          echo ::set-output name=tag::${TAG}

      - name: Set up Docker Buildx
        id: buildx
        # Use the action from the master, as we've seen some inconsistencies with @v1
        # Issue: https://github.com/docker/build-push-action/issues/286
        uses: docker/setup-buildx-action@master
        # Only worked for us with this option on 🤷‍♂️
        with:
          install: true
      
      - name: Cache Docker layers
        uses: actions/cache@v2
        with:
          path: /tmp/.buildx-cache
          # Key is named differently to avoid collision
          key: ${{ runner.os }}-multi-buildx-${{ github.sha }}
          restore-keys: |
            ${{ runner.os }}-multi-buildx

      - name: Build production image
        uses: docker/build-push-action@v2
        with:
          context: .
          builder: ${{ steps.buildx.outputs.name }}
          file: ./Dockerfile
          # Set the desired build target here
          target: app
          load: true
          tags: ${{ steps.prep.outputs.tagged_image }}
          cache-from: type=local,src=/tmp/.buildx-cache
          # Note the mode=max here
          # More: https://github.com/moby/buildkit#--export-cache-options
          # And: https://github.com/docker/buildx#--cache-tonametypetypekeyvalue
          cache-to: type=local,mode=max,dest=/tmp/.buildx-cache-new

      - name: Move cache
        run: |
          rm -rf /tmp/.buildx-cache
          mv /tmp/.buildx-cache-new /tmp/.buildx-cache

      - uses: superfly/flyctl-actions@master
        env:
          FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN }}
        with:
          args: deploy --config ./fly.prod.toml --image ${{ steps.prep.outputs.tagged_image }}