What's the purpose of "docker build --pull"?

Question

When building a docker image you normally use docker build ..

But I've found that you can specify --pull, so the whole command would look like docker build --pull .

I'm not sure about the purpose of --pull. Docker's official documentation says "Always attempt to pull a newer version of the image", and I'm not sure what this means in this context.

You use docker build to build a new image, and eventually publish it somewhere to a container registry. Why would you want to pull something that doesn't exist yet?

Answer 1

it will pull the latest version of any base image(s) instead of reusing whatever you already have tagged locally

take for instance an image based on a moving tag (such as ubuntu:bionic). upstream makes changes and rebuilds this periodically but you might have a months old image locally. docker will happily build against the old base. --pull will pull as a side effect so you build against the latest base image

it's ~usually a best practice to use it to get upstream security fixes as soon as possible (instead of using stale, potentially vulnerable images). though you have to trade off breaking changes (and if you use immutable tags then it doesn't make a difference)

Answer 2

Docker allows passing the --pull flag to docker build, e.g. docker build . --pull -t myimage. This is the recommended way to ensure that the build always uses the latest container image despite the version available locally. However one additional point worth mentioning:

To ensure that your build is completely rebuilt, including checking the base image for updates, use the following options when building:

--no-cache - This will force rebuilding of layers already available.

The full command will therefore look like this:

docker build . --pull --no-cache --tag myimage:version

The same options are available for docker-compose:

docker-compose build --no-cache --pull

Answer 3

Simple answer. docker build is used to build from a local dockerfile. docker pull is used to pull from docker hub. If you use docker build without a docker file it throws an error.

When you specify --pull or :latest docker will try to download the newest version (if any)

Basically, if you add --pull, it will try to pull the newest version each time it is run.