DOCKER.md: add named volumes to persist data

Add two volumes for the default Docker command:

1. terminus_storage: for data storage (the databases and the like)
2. terminus_config: for the config file

Both volumes persist when the container is stopped or even removed.

DOCKER.md

@@ -2,7 +2,9 @@
 
 1. Install Docker as described on the [Docker installation page](https://docs.docker.com/install/). Choose your
    operating system on the left sidebar.
-2. Open a command terminal (shell on Unix, Powershell or Cmd on Windows) and run `docker run --name terminus-server -p 6363:6363 -e SERVER_NAME=localhost -e DB_PASS=root -t -d terminusdb/terminus-server:latest`
+2. Open a command terminal (shell on Unix, Powershell or Cmd on Windows) and run `docker run -it --name terminus-server -d --mount type=volume,source=terminus_storage,dst=/app/terminusdb/storage --mount type=volume,source=terminus_config,dst=/app/terminusdb/config -p 6363:6363  --rm  terminusdb/terminus-server:latest`
+
+This will create two volumes: one to persist your database data and one to persist your configuration.
 
 You can configure the following environment variables to taste using the `-e ENV_NAME=value` in the docker command above: