adding notes about building the docker container

Author: luiscape

README.md

@@ -1,10 +1,14 @@
 ## MongoDB
 Runs the MongoDB container configured to use the local `/data` folder for its file storage.
 
+## Docker
+[![](https://badge.imagelayers.io/luiscape/rolltime-mongo:latest.svg)](https://imagelayers.io/?images=luiscape/rolltime-mongo:latest 'Get your own badge on imagelayers.io')
+
+A volume needs to be mounted in the container if the data needs to be persistent.
+
 ```
-docker run -v "$(pwd)":/data --name mongodb \
-  -e MONGODB_USER_NAME=rolltimedev \
-  -e MONGODB_USER_PASSWORD=rolltimedev \
-  -e MONGODB_DATABASE=rolltime \
-  -d mongo mongod --smallfiles
+docker run
+  -v ./data:/data \
+  --name mongodb \
+  -d luiscape/rolltime-mongo:latest
 ```