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"
}
}
]
}
```