validation: Only run tagged docker build once

validation: Only run tagged docker build once

Previously, we used to manually change the release tag for the Docker
container image(s) built for the validation project, let the build run
once, then create another change that disabled the job to prevent
further (useless) rebuilds happening each night.

Instead of triggering the docker build job for the release (tag) using
cron, switch to triggering the job based on gerrit merge events
affecting the ci-management repository and only if the said merge event
affects the 'validation-docker.yaml' configuration file (the one where
the release tag is defined/updated).

Change-Id: Ia4493b558ee7d1319c42a04690aa1c7580eb3731
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>