Building Docker Image

Sat 08 September 2018


Docker images are built from layered containers. The Dockerfile is a tool that lets you define reproducable docker image.

Generally, we are doing a few things inside a Dockerfile to define an image:

1. Basic Running Environment

Is your app running on Linux or Windows? Which version do you prefer?

2. Dependent Packages

What else packages does your app need? e.g. npm/nuget packages

3. Copy Application

Copy the generated binary files into the image.

4. Set up interfaces

Expose the port you want to listen, mounting point you want to monitor, etc.


Here is a simple proxy server that I'd like to move it into the container.

The App is simple HTTP proxy that attach CORS header for each request to a specific server. (Useful when you can't/don't want to update the API server)

var http = require('http')
var httpProxy = require('http-proxy');
var proxy = httpProxy.createServer({});
proxy.on('proxyRes', function (proxyRes, req, res) {
proxyRes.headers['Access-Control-Allow-Origin'] = '*'
var server = http.createServer(function (req, res) {
proxy.web(req, res, {
target: process.env.TARGET_URL

The code structure is like this:

├── index.js
├── node_modules
└── package.json

Now let's build an image for it.

Firstly, choose to use "Alpine Linux" as its the smallest Linux (5MB)

FROM alpine

Then install NodeJS and NPM.

RUN apk update && apk upgrade && apk add --update --no-cache nodejs npm

Now we can copy our source code into the image.

COPY proxy /proxy

To save the time after the container starts, we should use NPM-installed image.

RUN npm install

Lastly, make the Node command as our image entry point. Don't forget to expose port 80 so our app can listen to it.

ENTRYPOINT ["/bin/sh", "-c", "node /proxy/index.js"]

Now let's get them together to see how a Dockerfile looks like:

FROM alpine
RUN apk update && apk upgrade && apk add --update --no-cache nodejs npm
COPY proxy /proxy
RUN npm install
ENTRYPOINT ["/bin/sh", "-c", "node /proxy/index.js"]

Category: Container

Tagged: docker container