Upgrade all services for Traefik v2 ; clean Compose files ; update doc

pica-mattermost/docker-compose.yml

-version : "2.4"
+version : "3.7"
 networks:
-  docker_default:
+  proxy:
     external: true
-    name: "docker_default"
+  mattermost:
 
 volumes:
   mattermost-config:
@@ -30,12 +30,13 @@ services:
       - MM_SITEURL=https://team.picasoft.net
     env_file: ./secrets/mattermost-db.secrets
     labels:
-      - "traefik.frontend.rule=Host:team.picasoft.net"
-      - "traefik.port=8000"
-      - "traefik.frontend.passHostHeader=true"
-      - "traefik.enable=true"
+      traefik.http.routers.mattermost-app.entrypoints: websecure
+      traefik.http.routers.mattermost-app.rule: Host(`team.picasoft.net`)
+      traefik.http.services.mattermost-app.loadbalancer.server.port: 8000
+      traefik.enable: true
     networks:
-      - docker_default
+      - proxy
+      - mattermost
     depends_on:
       - mattermost-db
     restart: unless-stopped
@@ -48,5 +49,5 @@ services:
       - /etc/localtime:/etc/localtime:ro
     env_file: ./secrets/mattermost-db.secrets
     networks:
-      - docker_default
+      - mattermost
     restart: unless-stopped