Document exposing ports for docker-compose

CONTRIBUTING.md

@@ -88,9 +88,11 @@ However, you have to do some of the configuration yourself, because it's depende
 You can run a shell in a Docker container that pretty much should behave like a Debian distribution with:
 
 ```bash
-docker-compose run shell
+docker-compose run --service-ports shell
 ```
 
+(`--service-ports` exposes port 3000 so you can browse to the demo app on `localhost:3000`. If you just want to run a on-off command, or run the test suite, leave off the `--service-ports`.)
+
 The following instructions work for an Ubuntu host, and will probably work for other commong Linux distributions.
 
 Add a `docker-compose.override.yml` in the local directory, that looks like this:

docker-compose.yml

@@ -25,4 +25,6 @@ services:
  - WEBPACKER_DEV_SERVER_HOST=webpacker
  - WEB_CONCURRENCY=1
  - HISTFILE=/app/.bash_history
+ ports:
+ - "3000:3000"
  command: /bin/bash