Question 83351 in Docker Question posted in
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Building a container image and re-using pulled layers – Docker

Suppose there is a container image published to a Docker registry, and I want to rebuild the image with modifications while re-using as many of the published layers as possible.

Concretely, suppose the image foo/bar is built from this Dockerfile, and I only want to modify the layer that contains script.sh:

FROM ubuntu:focal
COPY script.sh .

Even though pulling the image downloads the layers for ubuntu:focal, when I re-build my local machine may resolve the ubuntu:focal tag to another version, producing a new image with no common layers with the one I pulled.

                                   6a9e8d7 <foo/bar:new>
           <foo/bar:old> c7632a5      |
                            |         |
                            +----+----+
                                 |
                              3b15784 <ubuntu:focal (then)>
                                 |
                                ...
           
           
                             DESIRABLE




        <foo/bar:old> c7632a5          48fead0 <foo/bar:new>
                         |                |
                         |                |
<ubuntu:focal (then)> 3b15784          9a634f5 <ubuntu:focal (now)>
                         |                |
                        ...              ...


                            UNDESIRABLE

The desired outcome could possibly be achieved by looking at the pulled layers and tagging the correct one (3b15784) as ubuntu:focal before building. But I’m not sure if Docker exposes enough information to do this in an automatic way.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    As a workaround, I could explicitly include the base image's digest as a label on the built image:

    FROM ${UBUNTU_IMAGE_ID:-ubuntu:focal}
COPY script.sh .

    Then I would build with:

    # First build
UBUNTU_IMAGE_ID=$( 
    docker inspect 
        --format '{{ index .RepoTags 0 }}@{{ index . "Id" }}' 
        ubuntu:focal 
)

# Subsequent builds
docker pull foo/bar:old
UBUNTU_IMAGE_ID=$( 
    docker inspect 
        --format '{{ index .Config.Labels "ubuntu_image_id" }}' 
        foo/bar:old 
)


docker build . 
    --build-arg "UBUNTU_IMAGE_ID=${UBUNTU_IMAGE_ID}" 
    --label "ubuntu_image_id=${UBUNTU_IMAGE_ID}" 
    --tag foo/bar:new

    However, a more elegant solution would definitely be welcome, especially one that does not involve any specific assumptions about how the original pulled image was built.


  2. 0

    Maybe Docker supports this built in with –cache-from:

    docker pull foo/bar:old
docker build . 
    --build-arg BUILDKIT_INLINE_CACHE=1 
    --cache-from foo/bar:old
    --tag foo/bar:new
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