Is it possible to extract the Dockerfile from a docker container

Question

I'm just starting out with Docker, and it would be very helpful to be able to see the Dockerfiles used to create existing docker images.

Even if the image was built by running commands manually, and then committing to a tag, it would be nice to be able to see how the image was made, both for learning purposes and for security.

Is there a way to extract a Dockerfile or list of commands used to build a given docker image?

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Edit (November 2021): Since people are still upvoting this, I can say that based on the answers and comments, I settled on:

docker history --no-trunc --format '{{.CreatedBy}}' <image> | grep -v '#(nop)' | tac 

It produces output that is easy to put in a Dockerfile. Example:

$ docker history --no-trunc --format '{{.CreatedBy}}' qemu | grep -v '#(nop)' | tac ENV DEBIAN_FRONTEND=noninteractive RUN /bin/sh -c apt-get update && apt-get -y upgrade # buildkit RUN /bin/sh -c apt install -y qemu-system-arm gcc-arm-none-eabi build-essential cmake bison flex # buildkit RUN /bin/sh -c useradd --create-home qemu # buildkit WORKDIR /home/qemu USER qemu COPY baremetal-arm baremetal-arm # buildkit 

But as I also wrote, I don't think there is a good way to extract the Dockerfile, so if you need it, and can't find the source code, maybe give the image a pass.

Answer 1

You have docker history <image> that is very helpful. It can even be used to generate a dockerfile if none of the steps involved stdin.

If a step as stdin, the only way to know what happened would be to do docker logs <container id parent>, but if you do not have the container, you can't.

Answer 2

There is undocker available now. We can install it by using the pip command.

pip install git+https://github.com/larsks/undocker/  

and use

docker save IMAGE_NAME | undocker -i -o OUTPUT_DIR 

to extract the files from docker.

https://github.com/larsks/undocker/