Suggestion: a "docker pull --dry-run" option

该 issue 未指定任何具体文件或测试。首先跟踪 Docker CLI 的 docker pull 命令入口点，并确定针对镜像可用性、返回状态、输出以及多平台镜像的预期 dry-run 行为。完成的标准是实现约定的行为并由测试覆盖。

Description

It would be useful in some automation environment to detect if there are new images available without pulling them right away. In my case, the launcher script detects if there is an update, and offers it to the user.

In an ideal world, I would launch docker pull --dry-run and either parse the output or check the return code.

Migrated from this issue, as suggested in the thread to make a feature request in the Docker CLI :
https://github.com/distribution/distribution/issues/2412

Workaround:

It is possible to achieve this using image inspection. The following script works in my case but has major flaws : is is slow, will break if the manifest structure changes, and works only because the variant I use happens to be the first in the remote repo's manifest. I feel that there is nothing better that a docker pull to know what a docker pull should be doing, and any scripting attempt like this one is an approximation.

    images=$(docker compose config | grep 'image:' | awk '{print $2}')
    for image in $images; do
        # Get the local digest in the format sha256:<hash>
        localDigest=$(docker image inspect --format='{{index .Id}}' $image 2>/dev/null)
        if [[ $? -ne 0 ]]; then
            echo "At least one image not found locally: $image"
            updatesFound=true
            break
        fi
        echo $localDigest

        # Get the remote digest in format sha256:<hash>
        # This works ONLY if the desired config is from the first variant
        # Would have been cleaner with jq but it is not available in a git bash on Windows...
        remoteDigest=$(docker manifest inspect -v $image \
            | awk '/"config":/ {found=1} found&& /"digest"/ {print $2; exit}' \
            | tr -d '",')
        echo $remoteDigest

        # Perform comparison
        if [[ "$localDigest" == "$remoteDigest" ]]; then
            echo "Up to date: $image"
        else
            echo "At least one image outdated: $image"
            updatesFound=true
            break
        fi
    done

EDIT: Workaround does not work on a mac, as the content of docker image inspect differs.