Copy several directories to another directory
You can do:
COPY ./ /code/It will copy everything from the current folder into the /code folder of your image.
So then you can create .dockerignore file to prevent of adding other files/directories then a, b and c. For example d, e and f are other directories in the current folder which should not be in the result image then content of the .dockerignore file will look like:
Dockerfile*defDisadvantage of this approach is that it will copy also .dockerignore into the /code folder.