[Task] Publish a `latest` tag for the Merlin base image

[karlhigley]

Description

Publish an additional tag on the Merlin base image that points to the most recently released version. (Not an additional image, just an additional tag that points at the same image.)

Additional context
This would allow us to update the nightly builds to use the latest tag, which would avoid the need to manually update the job configs after every release.

[karlhigley]

@jperez999 I think we're already doing this for some of the other repos, but I'm not sure how to do it with NGC