docker/frontend: Correctly proxy to backend

When using `npm run start --proxy http://backend:8888`, `npm` interprets
the `--proxy http://backend:8888` as a parameter that IT should parse,
which means the parameter never gets to `ember serve`. Add a `--` in the
middle so that `npm` knows not to parse that flag and instead pass it to
`ember serve`.

Author: bryanburgers

docker-compose.yml

@@ -37,7 +37,7 @@ services:
     build:
       context: .
       dockerfile: frontend.Dockerfile
-    entrypoint: npm run start --proxy http://backend:8888
+    entrypoint: npm run start -- --proxy http://backend:8888
     links:
       - backend
     ports: