add docker updating instructions

docker.md

@@ -31,3 +31,24 @@ docker compose up
 
 If you have done everything correctly, going to `http://localhost:8000` in your browser should show a question box screen. You may now log in with your admin password, and configure the instance.
 
+## Updating
+
+1. Stop the Docker container
+
+```sh
+docker compose down
+```
+
+2. Follow the [`UPDATE.md`](./UPDATE.md) file to see what to add or remove to your `config.json` or `.env`.
+
+3. Remove the Docker image
+
+```sh
+docker rmi catask_catask
+```
+
+4. Restart CatAsk
+
+```sh
+docker compose up
+```