Add opencontainers labels to the Grafana docker image

**Why is this needed**: By using the labels from Open Container Initiative it is possible to add metadata to the image to be used by other tools interacting/handling the image. **What would you like to be added**: Make the published docker image contain `org.opencontainers.image.source` label for easy reference back to this repository when the image gets updates. This is useful for tools like Renovate that makes updates to repositories when this image gets updates. OCI has standardized a few labels for adding metadata to images: https://github.com/opencontainers/image-spec/blob/main/annotations.md You have currently a label already, but it would have been nice if the mentioned `org.opencontainers.image.source` label were added, and preferable also the other standardized labels. Here is the current label you got: ```shell $ docker pull grafana/grafana:latest $ docker inspect grafana/grafana:latest --format '{{ json .Config.Labels }}' ``` Output: ```json {"maintainer":"Grafana Labs <hello@grafana.com>"} ``` **Who is this feature for?** Users of the Grafana docker container