Add npm install to frontend Docker

The frontend Docker image appears to be following a pretty standard Node
convention of adding the package.json, running `npm install`, and then
copying the rest of the application, which makes rebuilding new docker
images much faster if there are no dependency changes.

However, in the great `yarn` cleanup of 2017, `yarn install` was removed
but the corresponding `npm install` was not added back. This means that
when the frontend docker image tries to run ember, ember is not
available because it was never installed.

Author: bryanburgers

frontend.Dockerfile

@@ -1,7 +1,9 @@
 FROM node:8.6-alpine
 
 WORKDIR /app
-COPY package.json /app
+COPY package.json package-lock.json /app/
+
+RUN npm install
 
 COPY . /app