Push official images to Quay, in addition to Docker Hub

<!-- Please only use this template for submitting feature requests --> **What would you like to be added**: Hello everyone, Not sure if this is a 'feature' request, please feel free to relabel :) I would like to propose that we also push the official grafana/grafana container image to quay.io Please see the 'Why is this needed' section for a brief explanation on why. I'm happy to set up the quay repository and send a PR to the CI job that builds these images, once we have a general agreement from the maintainers that this is a good idea. **Why is this needed**: In a [recent blog](https://www.docker.com/blog/scaling-docker-to-serve-millions-more-developers-network-egress/), Docker hub announced rate limits on container pulls, in addition to many other changes to how image retention works. Since Grafana is one of the most widely used tools along with Prometheus, I think these rate limits would impact the production infrastructure at multiple organizations. The Prometheus images are already available on Quay, which is why it makes sense to also have the grafana/grafana image also published there. We currently mirror the image for our internal use-cases, but it would be much more awesome if we could get the official images pushed. Please let me know what you think about this :)