Attaches a com.centurylinklabs.watchtower.enable=false label to the
matrix-matrixto container by default, telling Watchtower (if in use)
to skip it. The image is built locally from source, so Watchtower
cannot update it and merely produces 'digest retrieval failed' errors
on every run.

A dedicated variable is used (instead of pre-filling
matrix_matrixto_container_labels_additional_labels) so that people
overriding the additional-labels variable do not lose the label.

Fixes #4820

Based on the report and initial patch in
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4821 by @der-domi

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>