CNG: Fix Docker labels

Clemens Beckrequested to merge
cb-fix-cng-docker-label into main

Fixes that Docker MRs miss all labels but the Docker one.

Example MR: gitlab-org/build/CNG!2035 (merged)

Use addLabels istead of labels to merge the Docker label with the default labels instead of overriding them.

Upstream documentation: https://docs.renovatebot.com/configuration-options/#addlabels

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