updated docker info

docker/readme.txt

@@ -30,3 +30,27 @@ docker run -p 4567:4567 apichallenges
 ```
 
 This will make the api challenges app accessible on `localhost:4567`
+
+To save your progress you need to mount a directory e.g. on
+
+Official Docker docks about mounting volumes https://docs.docker.com/storage/bind-mounts/
+
+- create a folder called `sessions` and save the user session files from the docker application in this folder.
+
+```
+docker run -p 4567:4567 apichallenges -v ".\sessions:/opt/app/challengersessions"
+```
+
+on linux/mac
+
+```
+docker run -p 4567:4567 apichallenges -v  "$(pwd)"/sessions:/opt/app/challengersessions"
+```
+
+Make a note of the container name and if you don't delete the container then you can restart it with:
+
+```
+docker container start name_of_container_here
+```
+
+Alternatively you could just restart the container from the docker desktop.