Build Cache

Definition

Docker's mechanism for dramatically speeding up the image building process by saving the result of every individual Dockerfile instruction as a reusable layer on the hard drive.

Explain Like I'm New

Docker takes notes. If step 2 is 'Install Python', Docker does it once and saves it. If you build the image again tomorrow, Docker says 'I already did step 2 yesterday', instantly skips it, and moves to step 3.

Real World Example

A developer changes one line of CSS in their app. Because they structured their Dockerfile correctly, Docker instantly uses the cached layers for installing Ubuntu, Node, and Webpack, and ONLY spends 2 seconds rebuilding the final CSS layer.

Common Use Cases

•Rapid prototyping
•CI/CD optimization

Interview Questions

basic

If you change a command on line 3 of your Dockerfile, will Docker still use the cached layer for line 4?

intermediate

Why do developers always run `COPY package.json` BEFORE they run `COPY . .` in Node.js Dockerfiles?

Flash Cards

Question

Will line 4 use cache?

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Answer

NO. The moment a single cache layer is 'busted' (changed), every single layer underneath it is instantly invalidated and forced to rebuild from scratch.

Question

Why copy package.json first?

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Answer

To protect the cache! `npm install` takes forever. If you `COPY . .` first, any tiny change to a CSS file will bust the cache, forcing `npm install` to run again. By copying ONLY the `package.json` first, `npm install` remains safely cached unless you actually change a dependency.