Build in Dockerfile

Is there any reason why that we don't do the build in the Dockerfile so that it gets built on `docker-compose build` rather than `docker-compose up`? By being in `up`, it effectively takes down your server every time you do a code push as there's a considerable wait for it to do the build inside the container. It also means that simple container restarts incur a build, even when you might not have pushed code.

In my project, I have changed the Dockerfile to:

```
FROM node:7
MAINTAINER jaga santagostino

RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

COPY package.json /usr/src/app
RUN npm install
COPY . /usr/src/app

ENV NODE_ENV production

# Build it
RUN npm run clean && npm run build && npm run build:server

EXPOSE 8000
CMD ["npm", "run", "start:prod"]
```

and added an additional volume map to `docker-compose.yml` to prevent volume-mapping the host's (blank) dist directory over top of what we've just built in the last step.

Regardez:

```
volumes:
- .:/usr/src/app/
- /usr/src/app/node_modules
- /usr/src/app/dist
```

Any reason for this? If not, I can do a PR as I think it's a much nicer workflow.