.dockerignore file located in subdirectory
You can not include by reference the .dockerignore file form the submodules.
The .dockerignore is usually related to a specific development tools and methods, so there is a good chance that the .dockerignore will already cover some of the major common files in the submodule as it covers in the main repository.
In case where there are discrepancies, you can add the content from the submodule .dockerignore to your main .dockerignore, corrected for the prefixes that are relevant to the submodule.