Register docker network info and use it for prometheus-node-exporter

Using the hardcoded IP did break while I was
messing with IPv6 stuff on the other branch

roles/matrix-base/tasks/setup_matrix_base.yml

@@ -23,6 +23,7 @@
   docker_network:
     name: "{{ matrix_docker_network }}"
     driver: bridge
+  register: matrix_docker_network_info
 
 - name: Ensure matrix-remove-all script created
   template: