How to properly use the Docker layer caching mechanism to speed up image builds?

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Effective use of the Docker build cache can reduce CI/CD pipeline waiting times from tens of minutes to just a few seconds. Each operator in a Dockerfile creates a separate immutable image layer. Docker intelligently analyzes each instruction and checks whether the context or the command itself has changed compared to the previous build. If everything remains the same, Docker simply takes the ready-made layer from the local or remote cache instead of executing the operation again.

The main secret to optimizing caching lies in the correct order of instructions. Since modifying any layer automatically invalidates the cache for all subsequent layers in the chain, commands should be ordered from least frequently changed to most frequently changed. For example, system dependencies and operating system packages are updated extremely rarely, while the application source code changes with every developer commit.

To get the maximum benefit from caching, it is recommended to follow this sequence of steps.

1. First, copy only dependency configuration files, such as package json or requirements txt, into the container.

2. Run project dependency installation so that this heavy layer is successfully cached.

3. Copy all the remaining application source code at later stages of the build.

4. Avoid using non-deterministic commands or dynamic arguments at the beginning of the file, as they break the cache.

By following these principles, you ensure that routine code changes do not trigger long re-downloads and reinstallations of third-party libraries, significantly speeding up the development and release delivery process.

How to properly use the Docker layer caching mechanism to speed up image builds?

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