Add docker multi-arch build (#55)

Add buildx & qemu for generating the multi-arch digest
Add builds for platforms amd64, arm64, Armv7 (Raspberry Pi 32bit), and 386

.github/workflows/docker-publish.yml

@@ -12,6 +12,7 @@ env:
   REGISTRY: ghcr.io
   # github.repository as <account>/<repo>
   IMAGE_NAME: ${{ github.repository }}
+  PLATFORMS: linux/amd64,linux/arm64,linux/arm/v7,linux/386
 
 
 jobs:
@@ -26,6 +27,12 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v3
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
       # Login against a Docker registry except on PR
       # https://github.com/docker/login-action
       - name: Log into registry ${{ env.REGISTRY }}
@@ -54,3 +61,4 @@ jobs:
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          platforms: ${{ env.PLATFORMS }}