Docker command/option to display or list the build context

Question

Is there a command/option to display or list the context which is sent to the Docker daemon for building an image?

$ docker build -t "image-name"
Sending build context to Docker daemon 8.499 MB
...

Files and directories can be excluded from the build context by specifying patterns in a .dockerignore file. I guess what I'm looking for amounts to testing the .dockerignore in addition to any other niche rules Docker uses when determined the context.

Answer 1

Answers above are great, but there is a low-tech solution for most cases - ncdu. This utility will show pretty and interactive tree structure with sizes. It has an option that will take patterns from a file and exclude them from scan. So you can just do ncdu -X .dockerignore. You will get something like this:

This is pretty close to what you will get in your docker image. One caveat is thou if you add a dot directory (like .yarn) into an image, it will not show in ncdu output.

Answer 2

The only way would be to add the current directory to an specific directory and list it.

Try building with this Dockerfile:

FROM busybox

RUN mkdir /tmp/build/
# Add context to /tmp/build/
COPY . /tmp/build/

Build it with:

docker build -t test .

List all the files and directories in /tmp/build:

docker run --rm -it test find /tmp/build

Answer 3

Starting with version 18.09, Docker has an option to export context data using BuildKit backend.

It's not enabled by default, so you need to set an environment variable DOCKER_BUILDKIT=1 before invoking docker build command.

The following command can work also if you don't have any Dockerfile in current directory.

printf 'FROM scratch\nCOPY . /' | DOCKER_BUILDKIT=1 docker build -f- -o context .

When you run multiple times remember to delete previous export with rm -r context.

You can also get context data as archive and then mount with archivemount command:

printf 'FROM scratch\nCOPY . /' | DOCKER_BUILDKIT=1 docker build -f- -o- . > context.tar
mkdir context
archivemount context.tar context

With both methods, then you can explore the result with ncdu context.

Answer 4

Updated answer: Since 2017, Docker has recommended to use COPY instead of ADD and with the comment from @tlrobinson, the simpler Dockerfile looks like so:

# debug and list the docker build context so that you can minimmize it
#
# usage:
#  docker build -f docker/context.Dockerfile -t test/buildcontext .
#
######################
FROM busybox

RUN mkdir /tmp/build/
# Add context to /tmp/build/
COPY . /tmp/build

# this last command outputs the list of files added to the build context:
RUN find /tmp/build/