What is Build Cache in Docker?

The Docker build cache is a mechanism that allows Docker to reuse previously created image layers during the build process. This feature significantly speeds up the build process by avoiding the need to re-execute commands that have not changed since the last build. Here’s a detailed explanation:

How Docker Build Cache Works

1. Layered Architecture: Docker images are built in layers, with each layer representing a change in the filesystem. Each instruction in a Dockerfile (e.g., RUN, COPY, ADD) creates a new layer.
2. Caching Mechanism: When Docker builds an image, it checks if a layer can be reused from the cache. If the instruction and the context (e.g., files being copied) have not changed, Docker will use the cached layer instead of rebuilding it.
3. Cache Invalidation: If any part of a layer changes, Docker invalidates that layer and all subsequent layers. For example, if a file copied into the image changes, Docker will re-execute the COPY command and all commands that follow it in the Dockerfile.

Benefits of Docker Build Cache

• Speed: Reusing layers from the cache can drastically reduce build times, especially for large images or images with many layers.
• Efficiency: It saves bandwidth and computational resources by avoiding redundant operations.
• Consistency: Ensures that unchanged parts of the image remain consistent across builds.

Best Practices for Using Docker Build Cache

• Order Instructions: Place frequently changing instructions (e.g., copying application code) towards the end of the Dockerfile to maximize cache reuse.
• Multi-Stage Builds: Use multi-stage builds to separate build and runtime dependencies, which can reduce the final image size and improve cache efficiency.
• Build Arguments: Use build arguments to force cache invalidation when necessary, ensuring that certain layers are rebuilt when specific conditions change.

Example

Consider the following Dockerfile:

FROM node:14

WORKDIR /app

COPY package.json yarn.lock ./
RUN yarn install

COPY . .
RUN yarn build

CMD ["node", "index.js"]

In this example:

• The COPY package.json yarn.lock ./ and RUN yarn install commands are likely...