Optimizing Docker Builds with .dockerignore

1. Understanding the Build Context

Before we can ignore files, we first need to understand what we're ignoring them from. When you run the docker build command, the first thing the Docker client does is package the files and directories specified in the command (e.g., the . in docker build .) into a tarball. This package is called the build context. The client then sends this entire context to the Docker daemon.

The problem is that without any filtering, this context includes everything in your project directory:

• Local dependencies like node_modules or Python's venv.
• Version control history in the .git directory.
• Temporary files, logs, and local configuration like .env.
• IDE settings folders like .vscode or .idea.

Sending all of this to the daemon is inefficient and can lead to security risks if sensitive files are accidentally copied into your image.

To get a formal definition and see the different ways a build context can be provided, please read the introductory sections of the official Docker documentation.

2. Introducing the .dockerignore File

The solution to the bloated build context problem is the .dockerignore file. It's a simple text file placed in the root of your build context (usually your project's root directory) that tells the Docker client which files and directories to exclude when creating the context tarball. Its syntax and behavior are intentionally similar to .gitignore, which you're likely already familiar with.

To see a practical demonstration of why this is necessary and how to create a basic .dockerignore file, let's watch a short video.

By excluding files, you gain several benefits:

• Faster Builds: A smaller context is sent to the daemon much more quickly.
• Smaller Images: Prevents accidental COPY-ing of unnecessary files into your image layers.
• Improved Security: Ensures that secrets, credentials, or local environment files are never included in the image.
• Avoiding Bugs: Prevents conflicts between local dependencies and dependencies installed within the container during the build.

3. .dockerignore Syntax and Patterns

While the basic concept is simple, the pattern-matching syntax offers a good deal of power and flexibility. Let's dive into the specifics.

A .dockerignore file is a list of patterns, with each line representing a pattern.

• Lines starting with # are comments.
• Wildcards like * (matches any sequence of non-separator characters) and ? (matches any single non-separator character) can be used.
• Docker also supports a special ** wildcard that matches any number of directories. For example, **/*.log will ignore any file with a .log extension in any subdirectory.
• You can create exceptions to exclusion rules by prefixing a pattern with !.

For a comprehensive guide to the syntax and some advanced examples, the official documentation is the best resource.

# Ignore all log files
*.log

# Ignore build artifacts
dist
target/

# Ignore documentation, but keep the main license
docs/
!docs/LICENSE

# Ignore all markdown files in the root
*.md

4. Practical Patterns for DevOps

As a DevOps engineer working with various technology stacks, having a set of ready-to-use patterns is invaluable. The following resource provides excellent, well-commented .dockerignore templates for common project types.

Here's a generic list of what you should almost always include in your .dockerignore file:

• .git
• .vscode / .idea
• node_modules
• npm-debug.log
• Dockerfile
• .dockerignore
• README.md
• .env

Excluding the Dockerfile and .dockerignore files themselves prevents them from being copied into the image with a broad COPY . . command, though they are still sent to the daemon so it can perform the build.

Conclusion

In this lesson, we took our first step towards creating production-grade Dockerfiles. You learned how to apply the .dockerignore file to optimize your build process.

Here are the key takeaways:

• The build context is the set of files sent to the Docker daemon to build an image.
• A .dockerignore file excludes specified files and directories from the build context, working much like .gitignore.
• Properly using .dockerignore results in faster builds, smaller images, and enhanced security by preventing secrets from being included in the image.
• The syntax supports comments (#), wildcards (*, ?, **), and exceptions (!), allowing for fine-grained control over the build context.

By keeping your build context lean, you establish a critical foundation for efficient and secure containerization.

In our next lesson, we'll build on this foundation by exploring another crucial optimization technique: leveraging the build cache effectively by ordering Dockerfile instructions. Now that we've optimized what goes into the build, we'll learn how to optimize the build process itself.