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Confused about image digests (lemmy.world)

submitted 7 months ago by thirdBreakfast@lemmy.world to c/docker@programming.dev

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I've been thinking about writing a script that would alert me if there was an updated version of an image I was running.

DockerHub shows an image digest on the page for that tag:

And I can extract the digest for an image I am running with:

docker inspect --format='{{index .RepoDigests 0}}' jc21/nginx-proxy-manager:latest

This matches the one from the DockerHub screenshot. But I can't see a CLI way to get the image digest from a registry. It seems like:

docker manifest inspect jc21/nginx-proxy-manager:latest

should do it, but it pulls out the digest of each of the architecture builds for that tag instead of the one shown in dockerhub.

Is there a way to compare the current local image with one in a registry from the command line? Or perhaps there's a more sensible way to do this?

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[–] thirdBreakfast@lemmy.world 2 points 7 months ago

For anyone coming to this later, digests are not presented in a super-understandable way (there's a good explanation of them in this stack-overflow). There appears to be two ways to get the digest that matches the one on the DockerHub page for the tag:

pull the image (what I was trying to avoid)
use API 2 (requires a token)

There's a good discussion of options for finding out if images have been updated in this r/SelfHosted discussion which is what I was trying to achieve.