Docker Image to Docker File - docker

Question

I have a docker image only.
Is it possible to get the Docker file that was used to build it?
If so, how?
Reason for that is that I loaded the image, so I don't have the Docker file that originated it.
Thanks!

Answer 1

There is a docker container(!) that does this (with some limitations), it is called dockerfile-from-image
https://github.com/CenturyLinkLabs/dockerfile-from-image
have a look at the (Ruby) code
https://github.com/CenturyLinkLabs/dockerfile-from-image/blob/master/dockerfile-from-image.rb
example launching this container to analyse itself
$ docker run --rm -v /run/docker.sock:/run/docker.sock centurylink/dockerfile-from-image
Usage: dockerfile-from-image.rb [options] <image_id>
-f, --full-tree Generate Dockerfile for all parent layers
-h, --help Show this message
and then if you launch it
$ docker run --rm -v /run/docker.sock:/run/docker.sock centurylink/dockerfile-from-image ruby
FROM buildpack-deps:latest
RUN useradd -g users user
RUN apt-get update && apt-get install -y bison procps
RUN apt-get update && apt-get install -y ruby
ADD dir:03090a5fdc5feb8b4f1d6a69214c37b5f6d653f5185cddb6bf7fd71e6ded561c in /usr/src/ruby
WORKDIR /usr/src/ruby
RUN chown -R user:users .
USER user
RUN autoconf && ./configure --disable-install-doc
RUN make -j"$(nproc)"
RUN make check
USER root
RUN apt-get purge -y ruby
RUN make install
RUN echo 'gem: --no-rdoc --no-ri' >> /.gemrc
RUN gem install bundler
ONBUILD ADD . /usr/src/app
ONBUILD WORKDIR /usr/src/app
ONBUILD RUN [ ! -e Gemfile ] || bundle install --system

Answer 2

You can view the commands were run to create each layer in an image - a sort of peek at the Dockerfile in effect - by running the following:-
docker history [IMAGE] | awk 'NR>1 {print $1}' | xargs docker inspect --format '{{ ((index .ContainerConfig.Cmd ) 0) }}'
If you just did a docker pull [IMAGE] then you can explore the Dockerfile in the standard repo:-
https://hub.docker.com/explore/
https://github.com/docker-library/official-images
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