CI: use a single job to publish multi-arch Docker images

Otherwise the earlier builds got overridden.

.github/workflows/publish-rolling.yml

@@ -42,10 +42,6 @@ jobs:
   docker:
     runs-on: ubuntu-20.04
 
-    strategy:
-      matrix:
-        platform: ['linux/amd64', 'linux/arm64', 'linux/arm/v7']
-
     steps:
       - uses: actions/checkout@v2
 
@@ -61,12 +57,12 @@ jobs:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
-      - name: Publish ${{ matrix.platform }} to Docker Hub
+      - name: Publish to Docker Hub
         uses: docker/build-push-action@v2
         with:
           context: .
           file: ./Dockerfile
-          platforms: ${{ matrix.platform }}
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
           push: true
           tags: |
             jesec/flood:master