ci: Enable multiplatform builds for amd64 and arm64 (#134)

* Add the 'platforms' argument to the docker/build-push-action GitHub
  Action and pass the platform names to enable build for both amd64 and
  arm64 architectures. This will enable Apple silicon machines to run
  the Docker images without emulation which will significantly improve
  runtime performance.

.github/workflows/docker.yml

@@ -100,6 +100,7 @@ jobs:
             org.opencontainers.image.created=${{ steps.prep.outputs.created }}
             org.opencontainers.image.revision=${{ github.sha }}
           push: true
+          platforms: linux/amd64,linux/arm64
 
       - name: Build and publish to registry with release tag
         if: github.event_name == 'release' && github.event.action == 'published' && github.repository == 'recast-hep/recast-atlas'
@@ -114,3 +115,4 @@ jobs:
             org.opencontainers.image.created=${{ steps.prep.outputs.created }}
             org.opencontainers.image.revision=${{ github.sha }}
           push: true
+          platforms: linux/amd64,linux/arm64