Docker command/option to display or list the build context

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.

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.

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

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.

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/

What worked for me is to do the following (based on this article).

1. Tell Docker to use the old build kit. In PowerShell that is:

   $ENV:DOCKER_BUILDKIT=0

2. Run Docker build so that it reports ALL the progress it's making:

   docker build --progress=plain BLAH

Given those two things you can then do something as simple as this in your Docker file:

RUN ls /app

And that will give you a list out of everything in the /app folder.