Missing manifest digest when building and pushing multi-platform images

The following build command results in images lacking manifests.

However, the cache image does have a manifest.
```shell
$ devcontainer build \
--workspace-folder . \
--cache-from registry.gitlab.com/jshbrntt/devcontainers/base/cache:latest \
--cache-to registry.gitlab.com/jshbrntt/devcontainers/base/cache:latest \
--platform linux/amd64 \
--platform linux/arm64 \
--image-name registry.gitlab.com/jshbrntt/devcontainers/base:latest
```

```shell
$ docker manifest inspect registry.gitlab.com/jshbrntt/devcontainers/base:latest
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 2000,
"digest": "sha256:392a7bfed82a8840bcde04a902dff2a4ead53ebfefd5f3dfdafdba0a4ed502e9",
"platform": {
"architecture": "amd64",
"os": "linux"
}
},
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 2000,
"digest": "sha256:e2837457d0638f4c5b759e19f331035fcdd494b44696a1051d24961b221ff0b5",
"platform": {
"architecture": "arm64",
"os": "linux"
}
},
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 566,
"digest": "sha256:57e6f2e1ad024393f23134b98db85ab518d6b3719f3efd562d6ab63c84f8b56d",
"platform": {
"architecture": "unknown",
"os": "unknown"
}
},
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 566,
"digest": "sha256:d8fcfd1cff48c696ad50c0fab56fc00d6a779c134a7f0cf175d95fc7d018ea1d",
"platform": {
"architecture": "unknown",
"os": "unknown"
}
}
]
}
```

Start by reproducing the provided devcontainer build command with the two platforms, then inspect the pushed image using docker manifest inspect and compare it with the cache image's manifest. Trace the CLI's multi-platform build-and-push entry point to determine where the published manifest differs. Done means the pushed image exposes the expected platform manifest digests without the reported missing-manifest behavior.

Written by the indexing model from the issue text.