Layers and cache

Learn about image layers and build cache.

Layers and the cache

Each instruction produces a layer and Docker caches the layers. If an instruction and its inputs (the copied files) didn't change, the layer comes from the cache (CACHED). When a layer changes, all layers after it are rebuilt:

 => CACHED [2/5] WORKDIR /app
 => CACHED [3/5] COPY requirements.txt .
 => CACHED [4/5] RUN pip install -r requirements.txt
 => [5/5] COPY . .                        # only the code changed

Turning off the cache

Sometimes the cache isn't wanted: when steps depending on an outside source like RUN apt-get update come from the cache, old packages can remain. --no-cache rebuilds all layers; --pull downloads a newer base image:

docker build --no-cache -t api:1.0 .
docker build --pull -t api:1.0 .
docker history api:1.0     # shows the layers and their sizes

Exercise Clean build and layers

Write two commands: build api:1.1 without the cache and pulling the newest base image; then show this image's layer history.

Show example solution

docker build --no-cache --pull -t api:1.1 .
docker history api:1.1

Test yourself

What happens if COPY . . is written before pip install?
- The packages are reinstalled on every code change
- The build fails
- The image shrinks
- The cache works better
Show answer

The packages are reinstalled on every code change — Everything after the changed layer runs again.

Build without using the cache.

docker build ____ -t api:1.0 .

Show answer

docker build --no-cache -t api:1.0 .

Match.

CACHED

Came from the cache
--no-cache

Rebuild all layers
--pull

Update the base image
docker history

Layer list

Next lesson: .dockerignore

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