build(ci): enhanced multi-platform build distribution example

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

build/ci/github-actions/multi-platform.md

@@ -58,18 +58,15 @@ In the previous example, each platform is built on the same runner which can
 take a long time depending on the number of platforms and your Dockerfile.
 
 To solve this issue you can use a matrix strategy to distribute the build for
-each platform across multiple runners and merge the resulting manifests into a
-single image using the [`buildx imagetools create` command](../../../engine/reference/commandline/buildx_imagetools_create.md).
+each platform across multiple runners and create manifest list using the
+[`buildx imagetools create` command](../../../engine/reference/commandline/buildx_imagetools_create.md).
 
 The following workflow will build the image for each platform on a dedicated
-runner using a matrix strategy and upload the resulting images as artifacts.
-Then, the `push` job will download the images, load them into the local Docker
-daemon, push them to a temporary local registry so Buildx can create a manifest
-list and push it to Docker Hub. It will copy the images from the local
-registry to Docker Hub as well.
+runner using a matrix strategy and push by digest. Then, the `merge` job will
+create a manifest list and push it to Docker Hub.
 
-Using a local registry is useful, so we don't create unecessary tags for each
-platform on the remote registry. Only `user/app:latest` will be created.
+This example also uses the [`metadata` action](https://github.com/docker/metadata-action)
+to set tags and labels.
 
 {% raw %}
 ```yaml
@@ -81,9 +78,7 @@ on:
       - "main"
 
 env:
-  TMP_LOCAL_IMAGE: localhost:5000/user/app
   REGISTRY_IMAGE: user/app
-  REGISTRY_TAG: latest
 
 jobs:
   build:
@@ -101,12 +96,11 @@ jobs:
         name: Checkout
         uses: actions/checkout@v3
       -
-        name: Prepare
-        run: |
-          mkdir -p /tmp/images
-          platform=${{ matrix.platform }}
-          echo "TARFILE=${platform//\//-}.tar" >> $GITHUB_ENV
-          echo "TAG=${{ env.TMP_LOCAL_IMAGE }}:${platform//\//-}" >> $GITHUB_ENV
+        name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ${{ env.REGISTRY_IMAGE }}
       -
         name: Set up QEMU
         uses: docker/setup-qemu-action@v2
@@ -114,48 +108,55 @@ jobs:
         name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
       -
-        name: Build
+        name: Login to Docker Hub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: Build and push by digest
+        id: build
         uses: docker/build-push-action@v4
         with:
           context: .
           platforms: ${{ matrix.platform }}
-          tags: ${{ env.TAG }}
-          outputs: type=docker,dest=/tmp/images/${{ env.TARFILE }}
+          labels: ${{ steps.meta.outputs.labels }}
+          outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
       -
-        name: Upload image
+        name: Export digest
+        run: |
+          mkdir -p /tmp/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "/tmp/digests/${digest#sha256:}"
+      -
+        name: Upload digest
         uses: actions/upload-artifact@v3
         with:
-          name: images
-          path: /tmp/images/${{ env.TARFILE }}
+          name: digests
+          path: /tmp/digests/*
           if-no-files-found: error
           retention-days: 1
   
-  push:
+  merge:
     runs-on: ubuntu-latest
     needs:
       - build
-    services:
-      registry:
-        image: registry:2
-        ports:
-          - 5000:5000
     steps:
       -
-        name: Download images
+        name: Download digests
         uses: actions/download-artifact@v3
         with:
-          name: images
-          path: /tmp/images
+          name: digests
+          path: /tmp/digests
       -
-        name: Load images
-        run: |
-          for image in /tmp/images/*.tar; do
-            docker load -i $image
-          done
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
       -
-        name: Push images to local registry
-        run: |
-          docker push -a ${{ env.TMP_LOCAL_IMAGE }}
+        name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ${{ env.REGISTRY_IMAGE }}
       -
         name: Login to Docker Hub
         uses: docker/login-action@v2
@@ -164,12 +165,13 @@ jobs:
           password: ${{ secrets.DOCKERHUB_TOKEN }}
       -
         name: Create manifest list and push
+        working-directory: /tmp/digests
         run: |
-          docker buildx imagetools create -t ${{ env.REGISTRY_IMAGE }}:${{ env.REGISTRY_TAG }} \
-            $(docker image ls --format '{{.Repository}}:{{.Tag}}' '${{ env.TMP_LOCAL_IMAGE }}' | tr '\n' ' ')
+          docker buildx imagetools create $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf '${{ env.REGISTRY_IMAGE }}@sha256:%s ' *)
       -
         name: Inspect image
         run: |
-          docker buildx imagetools inspect ${{ env.REGISTRY_IMAGE }}:${{ env.REGISTRY_TAG }}
+          docker buildx imagetools inspect ${{ env.REGISTRY_IMAGE }}:${{ steps.meta.outputs.version }}
 ```
 {% endraw %}