Use docker/metadata-action

This simplifies specifying tags and will also generate metadata labels for the image as per [OpenContainers spec](https://github.com/opencontainers/image-spec/blob/main/annotations.md)

.github/workflows/docker-release.yml

@@ -41,10 +41,21 @@ jobs:
           elif [ ${{ github.event.action }} == "release" ]; then
             echo "::set-output name=tags::${{ github.event.release.tag_name }}"
           else
-            echo "Unknown workflow trigger: ${{ github.event.action }}! Cannot determine tags."
+            echo "Unknown workflow trigger: ${{ github.event.action }}! Cannot determine default tag."
             exit 1
           fi
 
+      - name: Generate Docker image metadata
+        id: docker-meta
+        uses: docker/metadata-action@v4
+        with:
+          images: |
+            ${{ secrets.DOCKERHUB_USERNAME }}/${{ secrets.DOCKERHUB_REPO }}
+            ghcr.io/${{ github.repository_owner }}/${{ secrets.DOCKERHUB_REPO }}
+          tags: |
+            raw=${{ steps.tags.outputs.tags }}
+            raw=latest
+
       - name: setup platform emulator
         uses: docker/setup-qemu-action@v1
 
@@ -64,8 +75,5 @@ jobs:
           file: ./Dockerfile
           platforms: linux/amd64,linux/arm,linux/arm64/v8
           push: true
-          tags: |
+          tags: ${{ steps.docker-meta.outputs.tags }}
-            ${{ secrets.DOCKERHUB_USERNAME }}/${{ secrets.DOCKERHUB_REPO }}:latest
+          labels: ${{ steps.docker-meta.outputs.labels }}
-            ${{ secrets.DOCKERHUB_USERNAME }}/${{ secrets.DOCKERHUB_REPO }}:${{ steps.tags.outputs.tags }}
-            ghcr.io/${{ github.repository_owner }}/${{ secrets.DOCKERHUB_REPO }}:latest
-            ghcr.io/${{ github.repository_owner }}/${{ secrets.DOCKERHUB_REPO }}:${{ steps.tags.outputs.tags }}