Add instructions for PERSIST_DIR to grist-local-testing README

docker-compose-examples/grist-local-testing/README.md

@@ -9,4 +9,11 @@ See https://support.getgrist.com/self-managed for more information.
 
 ## How to run this example
 
-This example can be run with `docker compose up`.
+To run this example, change to the directory containing this example, and run:
+```sh
+docker compose up
+```
+Then you should be able to visit your local Grist instance at <http://localhost:8484>.
+
+This will start an instance that stores its documents and files in the `persist/` subdirectory.
+You can change this location using the `PERSIST_DIR` environment variable.

docker-compose-examples/grist-local-testing/docker-compose.yml

@@ -3,6 +3,6 @@ services:
     image: gristlabs/grist:latest
     volumes:
       # Where to store persistent data, such as documents.
-      - ${PERSIST_DIR}/grist:/persist
+      - ${PERSIST_DIR:-./persist}/grist:/persist
     ports:
       - 8484:8484