Trigger jobs for latest tag

Description

We would like to use the Trigger feature for making sure our container images are continuously updated with the latest patches for the OS. This could be achieved by triggering a CI pipeline say once a week. The issue is that we do not want to build master, but the latest release (ie latest tag).

Proposal

Add the ability on the trigger page to not only specify the branch or tag, but something more general like the latest tag or every tag (for example we want to patch both python:2 and python:3).