In this lesson, you will learn how to flatten your Docker images into a single layer to simplify distribution and reduce storage overhead.
The Concept of Squashing
As you have learned, Docker uses a layered filesystem where every RUN, COPY, or ADD instruction creates a new layer. While these layers are great for caching and deduplication, they can sometimes lead to "layer bloat," especially if you have many intermediate build files or temporary artifacts that aren't properly cleaned up.
Squashing is the process of taking all those individual layers and merging them into one single, unified layer. Think of it like taking a multi-page document—where each page represents a build step—and flattening it into a single high-resolution image.
Why Squash?
When you squash an image, you remove the metadata and the intermediate filesystem snapshots associated with each layer. This provides two main benefits:
- Simplified Distribution: The image registry sees only one blob to push or pull, which can streamline transfer processes.
- Reduced Complexity: It effectively hides the "how" of the build, leaving only the final state of the filesystem.
How it Looks
If you have a Dockerfile that performs several cleanup operations, like this:
FROM ubuntu:22.04
RUN apt-get update && apt-get install -y git
RUN rm -rf /var/lib/apt/lists/*
Even after the rm command, the files from the first RUN layer still exist in the image history. By squashing the image, the end result is just the final filesystem state. You are essentially taking a "snapshot" of the container after it is built and discarding the history of how you got there.
While multi-stage builds (which you learned previously) are generally the preferred way to keep images small by excluding build tools entirely, squashing is a powerful technique for deployments where you want a strictly flattened image structure without the complexity of managing multiple build stages.
Key Takeaway: Squashing flattens multiple image layers into one, simplifying image storage and removing unnecessary layer history from your deployment artifacts.