Why .dockerignore at the project root?

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1

I have a microservices project with two containers. The basic file layout is like this:

.
├── backend
│   ├── .dockerignore
│   ├── Dockerfile
│   └── app.py
├── docker-compose.yml
└── frontend
    ├── App.js
    └── Dockerfile

When I run balena push it complains:

[Warn]    The following .dockerignore file(s) will not be used:
[Warn]    * /home/ubuntu/notification-server/backend/.dockerignore
[Warn]    By default, only one .dockerignore file at the source folder (project
[Warn]    root) is used. Microservices (multicontainer) fleets may use a separate
[Warn]    .dockerignore file for each service with the --multi-dockerignore (-m)
[Warn]    option. See "balena help push" for more details.

I don’t understand this. I thought that each service would be built independently, and thus each should be able to have its own .dockerignore. Furthermore AFAIK the docker build process will actively refuse to consider anything outside of the root directory it’s building from - the so-called “context”. For security reasons, and layer caching reasons - it CAN NOT consider any files outside of the context.

So, why is the balena builder telling me to merge all the dockerignores from all my containers into one overarching one?