How do I create a Docker image from a container without configuration changes?



  • I am aware of two ways of creating Docker images:

    1. Build the image using a Dockerfile
    2. Use https://docs.docker.com/engine/reference/commandline/commit/ against a running container with changes.

    I find changing a Dockerfile each time I need image changes to be awfully inconvenient. Instead I have taken to launching ephemeral containers with docker run, installing packages I need on the fly with sudo, then committing the image.

    I didn't realize however there is a footgun involved in this approach, especially if you frequently use and switch up https://docs.docker.com/engine/reference/commandline/run/ hacks. I launch containers with changing bind mounts, environment variables, differing networks, etc. A lot of that seems to be considered part of the configuration of the container, and is committed in https://docs.docker.com/engine/reference/commandline/commit/ alongside actual filesystem changes.

    For example if you do:

    docker run --env FOO=BAR --name container_name ubuntu:focal
    docker commit container_name new_image
    docker run -it new_image
    echo $FOO
    

    You will see that the env variable FOO is now a part of new_image.

    This creates some awfully confusing situations, as I consider arguments to https://docs.docker.com/engine/reference/commandline/run/ to be as https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#create-ephemeral-containers as the container it creates. I am only concerned about persisting package installs.

    I would like to commit just the actual image, just the file system changes. Is there any way to do that?



  • Share us an example of your recurring "image changes". I think your problem is not with docker, is with the architecture of your application.

    • Data changes should be handled using a database not the file system.
    • Source code changes, should be managed with a git repository and the respective new build, generating a new image version ready to be deployed
    • Infrastructure changes should be ... That's what Docker was created for. You should modify the Dockerfile, push the changes, build a new image, etc
    • Configurations should be managed with environment variables manually injected at docker run or using some external service like consul, configurator, etc
    • If your application needs to upload or download files like images, reports, etc, you should use a specialized service like aws s3, gcp Filestore, some open source, etc

    As you can see, you don't need a huge native backup with docker commit.




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