Don't overwrite node_modules with Docker Compose

When creating the Dockerfile, we run `npm install`, which creates a
`node_modules` folder inside the working directory.

However, when using docker-compose, we overwrite the contents of that
working directory (/var/www/consul) with the contents of the host
machine's working directory. This means that, unless the `npm install`
command is run on the host machine to create a `node_modules` folder on
the host machine (which would pretty much defeat the point of using
Docker), the container won't have a `node_modules` folder and the
application won't run.

So we're defining a volume in docker-compose.yml to make sure we keep
the container's `node_modules` folder.

docker-compose.yml

@@ -33,8 +33,10 @@ services:
     volumes:
       - .:/var/www/consul
       - bundle:/usr/local/bundle
+      - node_modules:/var/www/consul/node_modules
     environment:
       - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
 volumes:
   db_data: {}
   bundle: {}
+  node_modules: {}