docker pull

Learn to pull images from a registry with docker pull.

docker pull

docker pull downloads an image from the registry. The image arrives in layers; layers already present locally aren't downloaded again (Already exists):

$ docker pull python:3.12-slim
3.12-slim: Pulling from library/python
a2318d6c47ec: Already exists
7f9a2c0b1d3e: Pull complete
Digest: sha256:9c4f…
Status: Downloaded newer image for python:3.12-slim

docker run pulls automatically if the image is missing; but it doesn't update an image that exists.

Platform and digest

Images are published for several processor architectures (amd64, arm64). Docker picks the one matching your machine; another architecture can be requested with --platform. Pulling by digest guarantees exactly the same content:

docker pull --platform linux/arm64 nginx:alpine
docker pull nginx@sha256:2d19…
docker run --pull always nginx:alpine     # check for the latest every time

Exercise Run with the current image

Write two commands: pull the python:3.12-slim image; then start a container named web in the background, checking the registry for the current nginx:alpine image every time.

Show example solution

docker pull python:3.12-slim
docker run -d --pull always --name web nginx:alpine

Test yourself

With an old nginx:alpine locally, what does docker run nginx:alpine do?
- It uses the old local image
- It always downloads the newest
- It errors
- It merges the two
Show answer

It uses the old local image — To update, docker pull or docker run --pull always is used.

Pull an image for the ARM architecture.

docker pull ____ linux/arm64 nginx:alpine

Show answer

docker pull --platform linux/arm64 nginx:alpine

Match.

Already exists

Layer exists locally
Pull complete

Layer downloaded
--platform

Architecture choice
@sha256:

Immutable content

Next lesson: docker push

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