Is there a way to add only changed files to a docker image as a new layer - without resorting to docker commit?

Question

TL;DR

Running COPY . /app on top of an image with but slightly outdated source code creates a new layer as large as the whole source code,

Answer 1

Actually, the solution IS to use COPY . /app as the OP is doing, there is however an open bug causing this not to work as expected on most systems

The only currently feasible workaround to this issue seems to be to use rsync to analyze the differences between the old and new images prior to pushing the new one, then use the changelog output to generate a tar-file containing the relevant changes which is subsequently COPY:ed to a new image layer.

This way, the layer sizes becomes a few bytes or kilobytes for smaller changes instead of 1.2 GB every time.

I put together documentation and scripts to help out with this over at https://github.com/neam/docker-diff-based-layers.

The end results are shown below:

Verify that basing the project images on the revision 1 image tag contents does not lead to desired outcome

Verify that subsequent COPY . /app commands re-adds all files in every layer instead of only the files that have changed:

docker history sample-project:revision-2

Output:

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
4a3115eaf267        3 seconds ago       /bin/sh -c #(nop) COPY dir:61d102421e6692b677   16.78 MB
d4b30af167f4        25 seconds ago      /bin/sh -c #(nop) COPY dir:68b8f374d8731b8ad8   16.78 MB
c898fe1daa44        2 minutes ago       /bin/sh -c apt-get update && apt-get install    10.77 MB
39a8a358844a        4 months ago        /bin/sh -c #(nop) CMD ["/bin/bash"]             0 B
b1dacad9c5c9        4 months ago        /bin/sh -c #(nop) ADD file:5afd8eec1dc1e7666d   125.1 MB

Even though we added/changed only a few bytes, all files are re-added and 16.78 MB is added to the total image size.

Also, the file(s) that we removed did not get removed.

Create an image with an optimized layer

export RESTRICT_DIFF_TO_PATH=/app
export OLD_IMAGE=sample-project:revision-1
export NEW_IMAGE=sample-project:revision-2
docker-compose -f rsync-image-diff.docker-compose.yml up
docker-compose -f shell.docker-compose.yml -f process-image-diff.docker-compose.yml up
cd output; docker build -t sample-project:revision-2-processed .; cd ..

Verify that the processed new image has smaller sized layers with the changes:

docker history sample-project:revision-2-processed

Output:

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
1920e750d362        24 seconds ago      /bin/sh -c if [ -s /.files-to-remove.list ];    0 B
1267bf926729        2 minutes ago       /bin/sh -c #(nop) ADD file:5021c627243e841a45   19 B
d04a2181b62a        2 minutes ago       /bin/sh -c #(nop) ADD file:14780990c926e673f2   264 B
d4b30af167f4        7 minutes ago       /bin/sh -c #(nop) COPY dir:68b8f374d8731b8ad8   16.78 MB
c898fe1daa44        9 minutes ago       /bin/sh -c apt-get update && apt-get install    10.77 MB
39a8a358844a        4 months ago        /bin/sh -c #(nop) CMD ["/bin/bash"]             0 B
b1dacad9c5c9        4 months ago        /bin/sh -c #(nop) ADD file:5afd8eec1dc1e7666d   125.1 MB

Verify that the processed new image contains the same contents as the original:

export RESTRICT_DIFF_TO_PATH=/app
export OLD_IMAGE=sample-project:revision-2
export NEW_IMAGE=sample-project:revision-2-processed
docker-compose -f rsync-image-diff.docker-compose.yml up

The output should indicate that there are no differences between the images/tags. Thus, the sample-project:revision-2-processed tag can now be pushed and deployed, leading to the same end result but without having to push an unnecessary 16.78M over the wire, leading to faster deploy cycles.