Create release-series tags in dockerhub

[paulmillar]

We have dCache images available from docker hub, with docker tags available for various releases.

A pattern I've seen with other software is to have a tag representing the latest bug-fix release of some series; for example, the tag 9.2 would be the latest 9.2 release of dCache. The latest 9.2 release is currently 9.2.38, so the 9.2 tag would point to the same image as the 9.2.38 tag. When 9.2.39 is released then 9.2 tag would be updated to point to tag 9.2.39.

It would be useful if the CI process that registers images and creates tags were to also update the corresponding series tag (e.g., 9.2).