I don't have a Docker Image - LocalOps Documentation

You are here because your awesome app is ready and now it’s time to package it.

Prerequisites

Your app
Docker to build standalone images to serve your production app.

Getting Started

Dockerizing your app ensures consistency, portability, isolation, and several other benefits. If you haven’t installed Docker already, you can find the installation instructions for various operating systems in the official docs. Let’s create a simple Node.js script that runs an HTTP server on port 3000 and when the URL / is hit, it prints Hello World!

app.js

const express = require('express');
const app = express();
const port = process.env.PORT ?? 3000;

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

We can run this app by running the following command:

node app.js

That’s it, we have our Node.js app running.

Writing the Dockerfile

Now, let’s create a Dockerfile for the above app. A Dockerfile is a set of instructions that tells Docker how to get our app working in our environment.

Dockerfile

# Use the official Node.js image with version 20 based on Alpine Linux
# Alpine images are smaller in size hence preferred
FROM node:20-alpine

# Set environment variable for the application's port
# this is used in app.js
ENV PORT=3000

# Set env to production to tell app and other tools that the app is running in production mode
# will be useful to disable debugging logs, skip few things etc.,
ENV NODE_ENV=production

# Set the working directory inside the container to /app
# this will be the root context of our app,
# we will be copying and running script from inside this folder
WORKDIR /app

# Copy package.json and package-lock.json to the working directory
COPY package.json package-lock.json* ./

# Install dependencies specified in package-lock.json
# using npm ci for a clean install
RUN npm ci

# Copy the rest of the application files to the working directory
# while copying this respects the contents given in dockerignore
COPY . .

# Expose the application port to be accessible from outside the container
EXPOSE ${PORT}

# Start the application
CMD ["node", "app.js"]

As you can see, the Dockerfile is a set of instructions that automate the steps you would take manually to get the app running on your machine. Though Docker supports Multi-Stage Builds, we won’t be using that here since Node.js needs all the source code and node_modules to be present while running the application. Prefer to opt-in for multistage builds whenever possible.

In real-world scenarios, you might have more than just an HTTP server. You might have other services running parallelly too, like schedulers, workers, notification services, gateways, etc,. You’ll need to create a Dockerfile for each and link them while creating the Helm charts.You might also choose to run everything in the same container, which is mostly not recommended, since you might lose isolation, scalability, etc.

It is heavily recommended to have a .dockerignore file to ignore certain prebuilt binaries and non-project-related stuff being copied from our local to the Docker container.

Read more on .dockerignore here. An example ignore file:

.dockerignore

Dockerfile
.dockerignore
node_modules
npm-debug.log
README.md
.git

It is essential to ignore folders like node_modules since when installing dependencies, few libraries might build binaries depending on your platform. We don’t want them to be copied since it might replace the binaries built inside the container itself while running npm ci, causing the application to break or misbehave.

Examples for Dockerfiles for Other Languages

GoLang

You can put any content in here, including other components, like code:This Dockerfile uses a multistage build to compile a Go application in a Golang builder image and then runs it in a lightweight Alpine image, ensuring a minimal final image size.

Dockerfile

# Stage 1: Build the Go application
FROM golang:1.18 as builder

# Set the working directory inside the container
WORKDIR /app

# Copy the Go module files and download dependencies
COPY go.mod go.sum ./
RUN go mod download

# Copy the rest of the application code
COPY . .

# Build the Go application
RUN go build -o main .

# Stage 2: Create a lightweight container to run the Go application
FROM alpine:latest

# Set the working directory inside the container
WORKDIR /root/

# Copy the Go binary from the builder stage
COPY --from=builder /app/main .

# Expose the application port
EXPOSE 8080

# Command to run the Go application
CMD ["./main"]

A .dockerignore file for reference:

# Ignore Go build artifacts
main
*.o
*.a
*.so
*.out
# Ignore other unnecessary files
.git
.gitignore
Dockerfile
docker-compose.yml

Django (Python)

This Dockerfile installs Django dependencies in one stage and then copies the dependencies and application code to a slim Python image, optimizing the final container size and performance.

Dockerfile

# Stage 1: Install dependencies
FROM python:3.10-slim as builder

# Set the working directory inside the container
WORKDIR /app

# Copy the requirements file and install dependencies
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the application code
COPY . .

# Stage 2: Create a lightweight container to run the Django application
FROM python:3.10-slim

# Set the working directory inside the container
WORKDIR /app

# Copy the installed dependencies and application code from the builder stage
COPY --from=builder /app /app

# Expose the application port
EXPOSE 8000

# Command to run the Django application
CMD ["python", "manage.py", "runserver", "0.0.0.0:8000"]

A .dockerignore file for reference:

# Ignore Python bytecode files
*.pyc
*.pyo
__pycache__
# Ignore other unnecessary files
.git
.gitignore
Dockerfile
docker-compose.yml
# Ignore local Django settings
local_settings.py
db.sqlite3

Ruby on Rails

This Dockerfile leverages a multistage build to install Ruby and system dependencies, precompile assets in a Ruby builder image, and then run the Rails application in a slimmer Ruby image, reducing the overall image size.

Dockerfile

# Stage 1: Build the Rails application
FROM ruby:3.1 as builder

# Set the working directory inside the container
WORKDIR /app

# Copy the Gemfile and Gemfile.lock and install dependencies
COPY Gemfile Gemfile.lock ./
RUN bundle install

# Copy the rest of the application code
COPY . .

# Precompile the Rails assets
RUN bundle exec rake assets:precompile

# Stage 2: Create a lightweight container to run the Rails application
FROM ruby:3.1-slim

# Set the working directory inside the container
WORKDIR /app

# Copy the installed dependencies and application code from the builder stage
COPY --from=builder /app /app

# Install system dependencies for Rails
RUN apt-get update && apt-get install -y nodejs

# Expose the application port
EXPOSE 3000

# Command to run the Rails application
CMD ["rails", "server", "-b", "0.0.0.0"]

A .dockerignore file for reference:

# Ignore Ruby build artifacts
*.gem
*.rbc
# Ignore other unnecessary files
.git
.gitignore
Dockerfile
docker-compose.yml
# Ignore local Rails settings
config/database.yml
db/*.sqlite3
log/*
tmp/*

All these Dockerfiles are references to how you might package the application; actual steps might vary depending on the application you build and the features you use.

Building Images

To build your Docker image, use the following command in the directory containing your Dockerfile:

docker build --platform linux/amd64 -t node-app:latest .

This command builds the node-app image for platform linux/amd64 and tags it as latest. If you are locally testing your application, you can skip the platform key to build the images

# only for testing in local machine
docker build -t node-app:latest .

Running Your Docker Image

Now, we have images, its time run the image. Use the following command:

docker run \
  -it \
  --rm \
  --name node-app \
  -e PORT=3000 \
  -d \
  -p 3000:3000 \
  node-app

-it: enables interactivity with TTY
--rm: to tell the Docker Daemon to clean up the container and remove the file system after the container exits
--name node-app: Name of the container node-app.
-e PORT=3000: Sets the environment variable PORT in docker 3000.
-d: Runs the container in detached(background) mode. You can skip the flag to see the logs directly in your terminal window.
-p 3000:3000: Maps port 3000 on your host to port 3000 in the container.
node-app in the end is the name of the image

This command runs your Docker image, mapping port 3000 of the container to port 3000 on your local machine. Once the service is up, visit http://localhost:3000 to see the app we built running. Hurray 🎉. Now we have package our app for production use using docker.

Next Steps

Once the docker is ready, the next steps will be

​Prerequisites

​Getting Started

​Writing the Dockerfile

​Examples for Dockerfiles for Other Languages

​Building Images

​Running Your Docker Image

​Next Steps

Prerequisites

Getting Started

Writing the Dockerfile

Examples for Dockerfiles for Other Languages

Building Images

Running Your Docker Image

Next Steps