Docker: copy contents from subdirectories not working

I have a very strange issue with copying the contents of subdirectories to a Docker container.
This is the directory structure:

enter image description here

Note: There are two Dockerfiles, I use the one on the upper level for test purposes. Ignore the one in the WebApp folder.

I want to copy the directories Bilder and JSON to the container, including all contents, but it doesn’t work. The folders in the container will be empty. However, copying the Testdir does indeed work.

This is part of my Dockerfile:

FROM python:3.7-buster

# -- Init --
RUN mkdir -p  /app/src
WORKDIR /app/src

ADD WebApp/Testdir ./Testdir #works
ADD WebApp/Bilder ./Bilder #doesn't work

CMD ["sleep", "50"] #to check contents

I build the image as part of a docker-compose.yml file with
docker-compose build test

Does anyone have a clue what’s happening here? I’ve been searching for a solution for quite some time…

Source: Docker Questions