Add mattermost-exporter to proxy docker network

pica-mattermost/docker-compose.yml

@@ -60,6 +60,7 @@ services:
     env_file: ./secrets/mattermost-exporter.secrets
     networks:
       - mattermost
+      - proxy
     labels:
       traefik.http.routers.mattermost-metrics.entrypoints: websecure
       traefik.http.routers.mattermost-metrics.rule: "Host(`team.picasoft.net`) && PathPrefix(`/metrics`)"