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Ask questionshelper image - encountered unknown type ; children may not be fetched

Context

We are building docker images on a CI agent, we have configured the daemon to use Nexus 3.16 as a registry mirror.

Docker version

server engine: 18.09.6
client: 18.09.2

Issue

When the agent is a newly provisioned one, we got the error below:

[internal] helper image for file operations
       digest: sha256:e4c43493b1bb2320466c52248e886014710025b273f224bb96a23b8fb7a1d193
         name: "[internal] helper image for file operations"
      started: ...
    completed: ...
     duration: 166.431518ms
        error: "encountered unknown type ; children may not be fetched"
 resolve docker.io/docker/dockerfile-copy:v0.1.9@sha256:e8f159d3f00786604b93c675ee2783f8dc194bb565e61ca5788f6a6e9d304061 0.2s done

But if we rerun the build job, it will pass. subsequent builds also pass.

We tried to find the dockerfile-copy image locally to reproduce the issue but we couldn't find it, so it means its stored/treated in a special way?

We got another team member who has never built this image with buildkit before to run the same script we run on CI on her local machine, and she could reproduce the issue.

Also, if we remove the registry mirror, it works fine.

Could anyone help explain why this is happening?

Thanks

moby/buildkit

Answer questions AkihiroSuda

Please try 19.03, which no longer pulls copy helper image

useful!

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