Test the ".dockerignore" file to ensure the build context doesn't contain unwanted files

dockerignore-test.sh

#!/bin/sh

# Based on BMitch's answer from:
# https://stackoverflow.com/questions/38946683/how-to-test-dockerignore-file

# Note: will create and delete temporary file "Dockerfile.build-context"

# 1. Copy to project folder where image is being built
# 2. Run script
# 3. You should see list of files in build context
# 4. If unwanted files in context, adjust .dockerignore file and go back to step 2

cat <<EOF > Dockerfile.build-context
FROM busybox
COPY . /build-context
WORKDIR /build-context
CMD find .
EOF

docker build -f Dockerfile.build-context -t build-context .
docker run --rm -it build-context

rm Dockerfile.build-context

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