How to test dockerignore file?

After reading the .dockerignore documentation, I'm wondering if there is a way to test it?

Examples

**/node_modules/

How do I check my dockerfile ignore the correct files and directories?

To expand on VonC's suggestion, here's a sample build command you can use to create an image with the current folder's build context:

docker image build --no-cache -t build-context -f - . <<EOF
FROM busybox
WORKDIR /build-context
COPY . .
CMD find .
EOF

Once created, run the container and inspect the contents of the /build-context directory which includes everything not excluded by the .dockerignore file:

# run the default find command
docker container run --rm build-context

# or inspect it from a shell using
docker container run --rm -it build-context /bin/sh

You can then cleanup with:

docker image rm build-context

To get a detailed analysis of the build context you could use [pwaller/docker-show-context][1].

$ go get -v -u github.com/pwaller/docker-show-context
$ cd ~/path/to/project/using/docker
$ docker-show-context

It outputs statistics about the build such as file sizes and upload times.

[1]: https://github.com/pwaller/docker-show-context "pwaller/docker-show-context"

To expand on BMitch's answer (which implements VonC's answer), you can do it with one simple command:

# no cache to ensure output
# progress plain in case you are using buildkit
docker build --no-cache --progress plain --file - . <<EOF
FROM busybox
COPY . /build-context
WORKDIR /build-context
RUN find .
EOF

Switching from CMD to RUN means you don't have to ever run the container as the build will output the result of the find ..

One way is to make a small Dockerfile with an ADD or COPY directive in it.

Try to add or copy a file in a node_modules folder: it is does not succeed, that would be because of the .dockerignore.

To expand on Lucas' answer which expands on BMitch's answer (which implements VonC's answer), if you are using buildx you will also need to set the type of progress output by adding --progress plain and of course to not forget the --no-cache:

docker build --no-cache --progress plain -f - . <<EOF
FROM busybox
COPY . /build_context
WORKDIR /build_context
RUN find .
EOF

One thing that the other answers do not consider, is that this will potentially copy many gigabytes of data and be very slow, when all you want to do is find out which file(s) you need to exclude to reduce the image size.

So here is how you test your .dockerignore without actually copying data:

$ rsync -avn . /dev/shm --exclude-from .dockerignore

What this will do, is try to sync your current directory with the empty in-memory folder /dev/shm verbosely and dry-run (don't actually copy anything) the --exclude-from option reads glob patterns in the same format as .gitignore and .dockerignore

You will end up with a list of files copied and a summary with the total size at the end:

file.bogus
tests/
tests/conftest.py
tests/test_model.py

sent 1,954 bytes  received 207 bytes  4,322.00 bytes/sec
total size is 209,916,337  speedup is 97,138.52 (DRY RUN)

Add it to .dockerignore:

*.bogus

and test again:

tests/
tests/conftest.py
tests/test_model.py

sent 1,925 bytes  received 204 bytes  4,258.00 bytes/sec
total size is 201,145  speedup is 94.48 (DRY RUN)

This is extremely fast and doesn't fill your disk.

Edit: There is one difference that I have found. For rsync the pattern *.bogus matches all files with that name regardless of the directory. .dockerignore however only matches *.bogus in the current directory. To get the same behavior you need to prefix the pattern with the path glob characters **/*.bogus This will still work with rsync.