Build Contexts

In this lesson, we'll explore what a build context is and how it influences the Docker build process. Understanding build contexts helps you optimize build times and reduce image size by sending only the necessary files to the Docker daemon.

What Is a Build Context?

The build context is the set of files and folders the Docker CLI packages and sends to the Docker daemon when running docker build. By default, Docker uses the current directory (.) as the build context.

docker build . -t my-custom-app

This command:

1. Archives everything under ..
2. Sends it to the Docker daemon.
3. Unpacks it into a temporary directory (e.g., /var/lib/docker/tmp/...).
4. Executes the instructions in your Dockerfile.

docker build .
# results in an image tagged: IMAGE_ID:latest

Example Dockerfile for a Flask App

FROM ubuntu

# Install Python and pip
RUN apt-get update && apt-get install -y python python-pip

# Install Flask and MySQL connector
RUN pip install flask flask-mysql

# Copy application source into the image
COPY . /opt/source-code

# Set environment variable and entrypoint
ENV FLASK_APP=/opt/source-code/app.py
ENTRYPOINT ["flask", "run"]

Specifying a Different Build Context

You can point Docker to any local directory containing your Dockerfile:

docker build /opt/my-custom-app -t my-custom-app

Docker will look for /opt/my-custom-app/Dockerfile and include all files under /opt/my-custom-app in the context.

Common Context Sources

Managing Context Size with .dockerignore

Sending large or unnecessary files (logs, build artifacts) can slow down builds, especially when the daemon is remote. To prevent this, create a .dockerignore file in your context root:

tmp
logs
build

Docker will exclude these paths when packaging the build context.

Remote Docker Daemon Output

When using a remote Docker daemon, you’ll see output similar to:

Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM ubuntu
...

This confirms the context has been sent over the network before the build steps execute.

Building from a Git Repository

Docker can directly use Git URLs as the build context:

# Clone the default branch
docker build https://github.com/myaccount/myapp

# Build a specific branch
docker build https://github.com/myaccount/myapp#feature-branch

# Build only a subfolder within the repo
docker build https://github.com/myaccount/myapp.git#docker

By default, Docker looks for Dockerfile at the root of the checked‐out code. Use -f to point to a different file:

docker build -f Dockerfile.dev https://github.com/myaccount/myapp

Summary

• The build context defines what files are sent to the Docker daemon.
• Use .dockerignore to exclude unnecessary files and speed up builds.
• You can build from local paths or Git repositories.
• The -f flag lets you specify a non-default Dockerfile.

Links and References

• Docker Build Reference
• Dockerignore Documentation