Save 100s of MBs with Docker multi-stage builds

If you're building a Docker image to serve a frontend that is compiled down to static files, you can save hundreds of MBs - and avoid some serious security issues - by excluding `node_modules` and everything else required to compile your Nodejs app by using multi-stage builds.

```docker
FROM node:15-something AS builder # use AS to define a name for this build stage

COPY    package*.json /app/
WORKDIR /app
RUN     npm install

COPY . /app/
RUN  npm run build # or whatever your build command is

# Now we start a new build stage
FROM nginx:1-something AS runtime

COPY --from=builder /app/dist/index.html /usr/share/nginx/html/ # Pay attention to the --from command which
COPY --from=builder /app/dist/static     /usr/share/nginx/html/ # which references the previous stage's name

WORKDIR /usr/share/nginx/html
```

Here's a before and after:

```shell
REPOSITORY   TAG     IMAGE ID     CREATED          SIZE
my_node_app  latest  2a0e6ea0c2fb 51 minutes ago   168MB
my_node_app  fatty   f262d57f5c62 4 hours ago      1.1 GB
```

The new image only has NGINX, and the static files it needs, and none of the Nodejs bloat required for compiling the app at all, and coming in at only *15%* of its original size!

gabrielfortuna
May 17, 2021