Don't attempt to docker push image manifests

[joshwlewis]

CI is failing here, with the following error:

+ docker push heroku/heroku:24.nightly
The push refers to repository [docker.io/heroku/heroku]
tag does not exist: heroku/heroku:24.nightly

That's because we're attempting to docker push a multi-arch image manifest. docker push supports images, not manifests. The equivalent command for a manifest list is docker manifest push, but we actually don't need that - the docker buildx manifest create command already publishes the image (as you can see here).