Publish both tagged and untagged images for releases
Publish both tagged and untagged images for releases

The Triggers release images are pushed to a global registry
(gcr.io/tekton-releases/) as well as regional registries (us.gcr.io,
asia.gcr.io, eu.gcr.io). The regional images were tagged with the
release version while the global one was not.

This commit ensure that the global image is tagged with the release
version as well. In addition, this also adds a `release.notags.yaml`
(just like Pipelines) to support container runtimes that do no support
the `tag@digest` notation.

Fixes #534

Signed-off-by: Dibyo Mukherjee <7d445be0c0af41c00c371a1f7b3669c3331901f5@google.com>

tektoncd/triggers,tektoncd/triggers