[Bug]: "Transport became inactive" on 'container build' (0.5.0) – 16k limit on Container-files?

### I have done the following

- [x] I have searched the existing issues
- [x] If possible, I've reproduced the issue using the 'main' branch of this project

### Steps to reproduce

The container-file at https://github.com/srcshelton/docker-gentoo-build/blob/master/gentoo-base/Dockerfile.env is 19k in size. Attempting to `container build` it results in:

```bash
$ /usr/local/bin/container build --file Dockerfile.env --tag localhost/gentoo-env:latest .
Error: unavailable (14): Transport became inactive
```

… but doing nothing other than removing commented lines (bringing the total size below 16k) proceeds to attempt to build the container image.

### Current behavior

Container-files of over 16k in length appear to fail to transfer to the `builder` `buildkit` instance.

### Expected behavior

Container-files of arbitrary length should be processed correctly (or, at least, a meaningful error message produced if any length-limit is exceeded).

### Environment

```markdown
- OS: macOS 26.0.1
- Xcode: Version 26.0.1 (17A400)
- Container: container CLI version 0.5.0 (build: release, commit: 48230f3)
```

### Relevant log output

```shell
N/A
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct