Add traefik.docker.network label to specify docker network exclusively

[tdgroot]

My Traefik log was get clogged up with the following warnings whenever a warden env stop/start/up/down was happening:

Could not find network named 'warden' for container '/magento2_varnish_1'! Maybe you're missing the project's prefix in the label? Defaulting to first available network." providerName=docker container=varnish-magento2-5e4efd55 serviceName=magento2-varnish
Could not find network named 'warden' for container '/magento2_php-fpm_1'! Maybe you're missing the project's prefix in the label? Defaulting to first available network." providerName=docker container=php-fpm-magento2-5e4efd55 serviceName=magento2-php-fpm
Could not find network named 'warden' for container '/magento2_elasticsearch-hq_1'! Maybe you're missing the project's prefix in the label? Defaulting to first available network." providerName=docker container=elasticsearch-hq-magento2-5e4efd55 serviceName=magento2-elasticsearch-hq
Could not find network named 'warden' for container '/magento2_elasticsearch_1'! Maybe you're missing the project's prefix in the label? Defaulting to first available network." providerName=docker container=elasticsearch-magento2-5e4efd55 serviceName=magento2-elasticsearch
Could not find network named 'warden' for container '/magento2_rabbitmq_1'! Maybe you're missing the project's prefix in the label? Defaulting to first available network." providerName=docker container=rabbitmq-magento2-5e4efd55 serviceName=magento2-rabbitmq

With this change, my Traefik log is less chaotic :).