Writing Your First Dockerfile

Learn how to create Docker images using Dockerfiles and understand the most common instructions.

A Dockerfile is a text file containing instructions used to build a Docker image.

Think of a Dockerfile as a recipe. Docker follows each instruction step-by-step and produces a reusable image that can be deployed anywhere.

What Is a Dockerfile?

A Dockerfile defines:

Example:

FROM node:20

WORKDIR /app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE 3000

CMD ["npm", "start"]

Every Dockerfile starts with a base image.

FROM node:20

Docker downloads the image if it does not already exist locally.

Sets the working directory inside the container.

WORKDIR /app

All future commands run from this location.

Copies files from your host machine into the image.

COPY . .

You can also copy specific files:

COPY package.json .

Executes commands while building the image.

RUN npm install

Common uses:

Documents which port the application uses.

EXPOSE 3000

This does not publish the port automatically.

Defines the default command executed when the container starts.

CMD ["npm", "start"]

Only one CMD instruction should exist in a Dockerfile.

Navigate to your project directory and run:

docker build -t my-app:v1 .

Docker reads the Dockerfile and creates an image.

Verify:

docker images

Start a container:

docker run -p 3000:3000 my-app:v1

Your application should now be accessible through your browser.

Avoid:

FROM node:latest

Prefer:

FROM node:20

Use lightweight base images when possible:

FROM node:20-alpine

Copy dependency files first:

COPY package*.json ./
RUN npm install

This improves rebuild performance.

Dockerfiles provide a repeatable way to package applications and dependencies into portable images that can run consistently across environments.