[Doc] Add volumes in template

template/docker-compose.yml

 version: "3.7"
 
 volumes:
-  # Name of Docker volume
+  # Declaration of Docker volume
   # Also use a name so that Docker Compose does not add
   # the current folder name
-  myvolume:
-    name: myvolume
+  # Use as many volumes as you want : one for
+  # data, one for configuration, etc.
+  data:
+    name: data
 
 networks:
   # Best pratice : put all services that do not need
@@ -32,6 +34,15 @@ services:
     networks:
       - docker_default
       - app
+    volumes:
+      # Mount the Docker Volume in the container
+      # to persist the data. The mount point
+      # is service-dependant.
+      - data:/mount_point
+      # Exemple of mounting a file for configuration.
+      # This is not a Docker volume, but a versionned
+      # file is this repository.
+      - ./config.json:/config.json
     # Don't put the .example extension, the real
     # file will be a copy with real values
     env_file: