Only publish Docker image on fresh tags or branch heads.

This is to make sure we do not accidentally re-publish already
existing tags. They can still be published manually by using the
manual publish job, if necessary.

Changelog: None

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

.gitlab-ci.yml

@@ -325,5 +325,5 @@ publish:docker-image:manual:
 
 publish:docker-image:automatic:
   rules:
-    - if: '$PUBLISH_MENDER_CONVERT_AUTOMATIC == "true"'
+    - if: '$CI_COMMIT_BRANCH =~ /^(master|staging|production|feature-.+|[0-9]+\.[0-9]+\.([0-9]+|x))$/'
   extends: .template:publish:docker-image