Enhancing Dgraph cd pipeline to only push images to Dockerhub when both AMD and ARM builds are successful

Dgraph CD pipeline was prematurely publishing Docker images to DockerHub even when one of the architectures failed. This PR makes the following changes to prevent this:

dgraph-build-amd64 and dgraph-build-arm64 jobs build their respective images. But do not push them to DockerHub.
Job dgraph-docker-manifest is renamed to dgraph-docker-image-and-manifests-push to reflect that it also pushes docker images for Dgraph/Custom/Standalone

dgraph-io / dgraph

Enhancing Dgraph cd pipeline to only push images to Dockerhub when both AMD and ARM builds are successful #9202