CI: push multiarch Docker images

This commit modifies the CI configuration so that the containers that
are built are actually manifest lists with images for multiple
architectures, not just AMD64. This is helpful for modern clouds and
hybrid clusters, where it's possible that not all machines are AMD64.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

Author: squat

.github/workflows/build.yaml

@@ -5,6 +5,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
       - name: Login to Docker Hub
@@ -24,6 +26,7 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          platforms: linux/arm64, linux/arm, linux/amd64
   snapshot:
     runs-on: ubuntu-latest
     steps: