Swap out a layer of a container image

Question

I am wondering if it is possible to swap out a layer of a container image for another. Here is my scenario:

I have a docker file that does the following:

1. It pulls the .net core 3.1 debian slim (buster) image
2. Builds the source code of my application and adds it as a layer on top of the .net core image

When a new version of the .net core 3.1 runtime image comes out, I would like to make a new image that has that new version, but has the same application layer on top of it.

I do NOT want to have to find the exact version of the code I used to build the application and re-build.

The idea is to replicate upgrading the machine and runtime, but not have any alteration to the application (less to test with the upgrade).

Is there a docker command that I can use to swap out a layer of an image?

Answer 1

This answer is broken into two distinct sections:

• Part I: Build From Sources Instead: Explains why this idea should not be done.
• Part II: A Proof-of-concept Implemented CLI Tool: A CLI tool I implemented specifically for this question, with a demonstration.

The first section can be skipped if potentially adverse side effects are not a concern.

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Part I: Build From Sources Instead

No, there is probably not a docker command for this - and likely never will, due to a whole slough of technical issues. Even by design, this is not meant to be trivially possible; images and layers are meant to be immutable after being built. It is strongly recommended that the application image is instead rebuilt from the original sources, but with a new base image by modifying the FROM command.

There are numerous consistency issues that make this idea ill-advised, of which a few are listed below:

• Certain Dockerfile commands do not actually create a layer, they update the final layer's internal manifest and the image's manifest. Commands that state Removing intermediate container XXXXXXXXXXXX are actually updating these aforementioned manifests, and not creating new layers. This requires correctly updating the only relevant changes when swapping from the old base image to the new base image; e.g. reconciling changes from ENV/LABEL/MAINTAINER/EXPOSE/CMD/ENTRYPOINT commands.

• ENV commands that alter the application image's configuration from variables inherited in the previous base image may not be updated correctly. For example, in the application image, there might be the following command ENV command:

  ENV PATH="/path:${PATH}"
  

  If the application image's old base image layers are swapped out, and the new base image contains a different ${PATH} variable, it is ambiguous how to reconcile the differences without a manual decision by a developer.

• If apt/apt-get/apk/yum is used to install Linux packages, these packages are installed in the application image as subsequent layers. The installed packages are not guaranteed to be compatible with the new base image if the underlying OS or executables change in the new base image layers.

Part II: A Proof-of-concept Implemented CLI Tool

"Upgrading" an image's base is technically possible by doing direct manipulation on the image archive of the already-built Docker application image. However, I cannot reiterate this enough - you should not even be attempting to edit the bottom layers of an existing image. ^{_{Seriously - stop it, get some help. I am 90% sure this is a war crime.}}

For the sake of humoring this question thoroughly though, I have developed a proof-of-concept CLI tool written in Java over on my GitHub project concision/docker-base-image-swapper that is designed to swap base images (but not arbitrary layers). The design choices I made to resolve various consistency issues are a "best guess" on what action should be taken.

Included is a demonstration for swapping the base image for an already-built Java application image from JDK8 to JDK11, implemented in the demo/demo.sh script. All core code is ran in isolated Docker contains, so only Bash and Docker are necessary dependencies on the host to run this demonstration. The demo application image is only built once on JDK 8, but is run twice - once on the original JDK 8 image, and another time on a swapped-base JDK 11 image.

If you experience some technical difficulty with the tool, I may potentially be able to fix the issue. This project was quickly hacked together and has pretty poor code quality; furthermore, it likely suffers from various unaccounted edge cases. I might thoroughly rewrite this in Rust within the next month or two with a focus on maintainability and handling all edge cases.

Warning: I still do not advise trying to edit images; the use of the tool is at your own risk.

The Concept

There are three images relevant in this process, all of which are already-built (i.e. no original sources are needed):

• application image: the application image that is currently based off of the old base image, but needs to be swapped to a newer base image.
• old base image: the older base image that the application image is based off using a FROM command in the original Dockerfile.
• new base image: the newer base image that should replace the application image's layers inherited solely from the FROM old-base-image layers.

By knowing which layers and configurations of the application image are inherited from the old base image, they can be replaced with the layers and configurations from the new base image. All image layers and manifests can be obtained as a tar archive by using the docker save command. With an archive(s) of all relevant three images, a tool can analyze the differences between

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Warnings on Alternatives

Beware of doing simply a COPY --from=... from the old application image, as the original application's image configuration (through commands such as CMD, ENTRYPOINT, ENV, EXPOSE, LABEL, USER, VOLUME, WORKDIR) will not be properly replicated.

Answer 2

Is it possible? Yes. But it's rarely done because of how error prone it is. For example, if the previous build would have created a library, but the library already exists in the original base image, that library won't be included in the first build. If the base image removes that library, then the resulting merged image will be missing that library since it's not in the new base layers and isn't in the old application layer.

There are a few ways I can think of to do this.

Option 1: If you know the specific files in one image, then use the COPY --from syntax to copy those files between images. This is the least error prone method, but requires that you know every file you want to include. The resulting Dockerfile looks like:

FROM new_base

# if every file is in /usr/local/bin:
COPY --from=old_image /usr/local/bin/ /usr/local/bin/

Option 2: you can export the images and create your own new image by combining the layers between the two. For this, there's docker save and docker load. That would look like:

docker save old_image >old_image.tar
docker save new_base >new_base.tar
mkdir old_image new_base new_image
tar -xvf old_image.tar -C old_image
tar -xvf new_base.tar -C new_base
cp old_image/*.json new_image/
# manually: copy each layer directory you want to save from old_image, you can view the nested tar files to find the proper ones
# manually: copy each layer directory from new_base into new_image
# manually: modify the new_image/manifest.json to have the new tag for your image, and adjust the layer references
tar -cvf new_image.tar -C new_image .
docker load <new_image.tar

Option 3: this could be done directly to the registry with API calls. You would pull the old manifest, adjust it with the new layers, and then push any new layers and then the new manifest. This would require a fair bit of coding (see regclient/regclient for how I've implemented some of these API's in Go).

Option 4: I know I've seen a tool that does this in a specific scenario but the name of it escapes me. I believe it required that you use their base images that were curated to reduce the risk of incompatibilities between versions, and limited what base images could be swapped, so I don't think it was a general purpose tool.

Note that option 2 and 3 both require either manual steps or for you to write some code if you want to automate it. Because of how error prone this is (as described above) I don't think you'll find anyone maintaining and supporting a tool to implement it. The vast majority rebuild from a Dockerfile using a CI tool.