Build context concept in Docker - Commands & Configuration

Introduction

When you build a Docker image, Docker needs to know which files to include. The build context is the folder and its contents that Docker uses to find files during the build. It helps Docker copy files into the image and run commands on them.

When you want to include your app files inside a Docker image

When you need to copy configuration files into the image during build

When you want to run commands on files that are part of your project during image creation

When you want to avoid sending unnecessary files to Docker to speed up the build

When you want to organize your Dockerfile and related files in one folder

Commands

This command builds a Docker image named 'my-app-image' using the folder './app' as the build context. Docker will look inside './app' for the Dockerfile and any files needed during the build.

Terminal

docker build -t my-app-image ./app

Expected Output

Sending build context to Docker daemon  4.096kB

Step 1/3 : FROM alpine:3.18

3.18: Pulling from library/alpine

Digest: sha256:...

Status: Downloaded newer image for alpine:3.18

 ---> 3f53bb00af94

Step 2/3 : COPY . /app

 ---> Using cache

 ---> 7a8b9c1d2e3f

Step 3/3 : CMD ["/app/start.sh"]

 ---> Running in 9c8d7e6f5a4b

Removing intermediate container 9c8d7e6f5a4b

 ---> 1a2b3c4d5e6f

Successfully built 1a2b3c4d5e6f

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    1a2b3c4d5e6f   10 seconds ago   5.6MB

Key Concept

The build context is the folder Docker uses to find files during image build; only files inside this folder can be accessed in the Dockerfile.

Common Mistakes

Running 'docker build' with the wrong folder as context

Docker cannot find the Dockerfile or needed files, causing build errors.

Always specify the folder containing your Dockerfile and related files as the build context.

Including large unnecessary files in the build context

This slows down the build because Docker sends all context files to the daemon.

Use a .dockerignore file to exclude files and folders not needed for the build.

Summary

The build context is the folder Docker uses to find files during image build.

Use 'docker build -t image-name path' where 'path' is the build context folder.

Keep the build context small to speed up builds and avoid errors.