Using .dockerignore - Commands & Configuration

Introduction

When building Docker images, unnecessary files can make the image bigger and slower to build. The .dockerignore file helps you tell Docker which files or folders to skip during the build, making the process faster and the image smaller.

When you want to exclude local files like logs or temporary files from your Docker image.

When your project has large folders like node_modules or build artifacts that are not needed inside the container.

When you want to speed up Docker builds by avoiding copying unnecessary files.

When you want to keep sensitive files like environment configs out of the Docker image.

When you want to reduce the size of the final Docker image for faster deployment.

Config File - .dockerignore

.dockerignore

node_modules
*.log
.git
.env
build/

This .dockerignore file tells Docker to ignore the node_modules folder, all files ending with .log, the .git folder, the .env file, and the build folder during the image build.

This keeps the image clean and small by excluding files that are not needed inside the container.

Commands

This command builds a Docker image named my-app-image using the current folder as the build context. Docker will use the .dockerignore file to skip unwanted files.

Terminal

docker build -t my-app-image .

Expected Output

[+] Building 5.3s (8/8) FINISHED

 => [internal] load build definition from Dockerfile 0.1s

 => [internal] load .dockerignore 0.0s

 => [internal] load metadata for docker.io/library/node:18 1.2s

 => [1/4] FROM docker.io/library/node:18@sha256:... 0.0s

 => [2/4] WORKDIR /app 0.0s

 => [3/4] COPY package.json package-lock.json ./ 0.5s

 => [4/4] RUN npm install 3.3s

 => exporting to image 0.1s

 => => exporting layers 0.1s

 => => writing image sha256:... 0.0s

Successfully built sha256:...

Successfully tagged my-app-image:latest

→

-t - Assigns a name and optionally a tag to the image

This command lists the Docker images named my-app-image to verify the image was built successfully.

Terminal

docker images my-app-image

Expected Output

REPOSITORY      TAG       IMAGE ID       CREATED          SIZE

my-app-image    latest    7a8b9c0d1e2f   10 seconds ago   350MB

Key Concept

If you remember nothing else from this pattern, remember: the .dockerignore file tells Docker which files to skip during build, making your images smaller and builds faster.

Common Mistakes

Not creating a .dockerignore file and copying all files including large or sensitive ones.

This makes the Docker image unnecessarily large and may expose sensitive data.

Create a .dockerignore file listing files and folders to exclude from the build context.

Using incorrect patterns in .dockerignore that do not match the files to exclude.

Docker will still copy unwanted files, defeating the purpose of .dockerignore.

Use correct syntax and test patterns to ensure the right files are ignored.

Summary

The .dockerignore file excludes files and folders from the Docker build context.

Use docker build with the current folder; Docker reads .dockerignore automatically.

Check the built image with docker images to confirm successful build.