Split build image actions (#17793)

* Update build-image.yml

* Update build-image.yml

* Update build-image.yml

* Update build-image.yml

.github/workflows/build-image.yml

@@ -13,6 +13,9 @@ on:
 jobs:
   build-image:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [linux/arm64, linux/amd64]
     steps:
       - uses: actions/checkout@v2
       - uses: docker/setup-qemu-action@v1
@@ -35,7 +38,7 @@ jobs:
       - uses: docker/build-push-action@v2
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
+          platforms: ${{ matrix.os }}
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           cache-from: type=registry,ref=tootsuite/mastodon:latest