ci(gha): enable arm64 docker builds

Author: joemiller

.github/workflows/build.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
       - main
+      - joem/ci-arm64 # TODO temporary
 
 env:
   IMAGE_NAME: ${{ github.repository }}
@@ -37,13 +38,19 @@ jobs:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           tags: type=sha
 
+      # setup qemu and buildx for cross-builds (arm64)
+      - name: Set up QEMU (for arm64 builds)
+        uses: docker/setup-qemu-action@49b3bc8e6bdd4a60e6116a5414239cba5943d3cf # v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@988b5a0280414f521da01fcc63a27aeeb4b104db # v3
+
       - id: push
         name: Build and push Docker image
         uses: docker/build-push-action@v6
         with:
           context: .
           labels: ${{ steps.meta.outputs.labels }}
-          #platforms: linux/amd64,linux/arm64 # TODO support ARM, too, with this and a different "driver"
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}