Using .Dockerignore file

We know that we can run our docker images on cloud services which provide high computations at low cost. However, one might wonder why we need to optimize a docker image. Think of a situation where you have copied a large file in your docker container that you actually don't need. It will obviously increase the size of the docker image, increase the overall build time, and cause caching issues. So, why not use a simple technique to avoid all these issues and improve the overall performance of the docker build process.

What is a .dockerignore file?

Similar to a .gitignore file, a .dockerignore file allows you to specify a list of files and/or directories which you want to ignore while building the image. This reduces the size of the image and helps speed up the docker build process. To understand this process, we first need to understand what Docker build context is.

What is Docker Build Context?

Docker is basically a Client-Server application. It has a docker client which runs on your local machine through command line tools and a docker server (also known as docker daemon). The Docker client needs to communicate with the Docker server to carry out instructions like building a docker image. The docker server can run on a remote machine, local machine, or in the cloud.

When we build a docker image, we need to send files to the docker server. These files constitute the build context. The files are archived into a .tar file by the docker client and then uploaded to the docker server.

How Does .dockerignore Help?

Before sending files to the docker server, the docker client searches for a file called .dockerignore in the build context's root directory. If found, the CLI modifies the build context to exclude files and directories mentioned in the .dockerignore file.

Benefits of Using .dockerignore

Reduces Image Size

If the build context is smaller, the resulting docker image will be smaller. Uploading smaller images to cloud services or transferring them to remote servers is less costly and faster compared to large images.

Cache Invalidation Prevention

The most common practice is to copy the entire directory to create an image using:

WORKDIR /usr/src/app
COPY . .

If we make changes in the build context, the build process creates a new image layer every time, which is expensive for large images. Any change leads to cache invalidation and breaks the cache for following commands. Removing large unnecessary files from the build context prevents this issue.

Security Protection

You might have important source files, secret keys, or passphrases in your build context that shouldn't be exposed in the final docker image. For example, exposing your .git folder inside your docker image creates security risks. Always ignore such sensitive files by mentioning them in the .dockerignore file.

How to Create a .dockerignore File

Here are examples of how to specify files, folders, and directories in your .dockerignore file:

# Ignore the logs directory
logs/

# Ignoring the password file
passwords.txt

# Ignoring git and cache folders
.git
.cache

# Ignoring all markdown and class files
*.md
**/*.class

# Ignore node_modules
node_modules/

# Ignore temporary files
tmp/
*.tmp
*.log

Common Patterns

Should You Ignore the Dockerfile?

Whether to ignore the Dockerfile depends on your preference. If you want users to understand how your docker image is built, don't add it to .dockerignore. If you want to keep the build process private, you can include it. However, most developers keep the Dockerfile visible for transparency and debugging purposes.

Conclusion

Creating a .dockerignore file in your build context is always recommended to make docker images smaller, more secure, and build processes faster. It's a simple yet powerful tool that prevents unnecessary files from bloating your containers and exposing sensitive information.