Fails with `# syntax=devthefuture-org/dockerfile-x`

Hi there,

In my case, I fork an open source project which defines its own devcontainer Dockerfile.

However, I need to make changes to it. If I make my changes directly in the Dockerfile, every time I pull from this upstream repository I will have merge conflicts to resolve.

I wanted to use https://github.com/devthefuture-org/dockerfile-x#from to avoid this.

Here's how it works:

- `.devcontainer/Dockerfile`

```dockerfile
FROM ubuntu
```

- `.devcontainer/fork.Dockerfile`

```dockerfile
# syntax = devthefuture/dockerfile-x

FROM ./Dockerfile

RUN echo add my customizations
```

And it works great when using Docker CLI with `docker build .devcontainer -f .devcontainer/fork.Dockerfile`.

But `devcontainer build` fails with:

```console
❯ devcontainer build --workspace-folder .
[14 ms] @devcontainers/cli 0.75.0. Node.js v22.14.0. linux 6.8.0-52-generic x64.
[351 ms] Path './dockerfile' for input 'docker.io/./dockerfile' failed validation. Expected path to match regex '/^[a-z0-9]+([._-][a-z0-9]+)*(\/[a-z0-9]+([._-][a-z0-9]+)*)*$/'.
invalid reference format: repository name (Dockerfile) must be lowercase
{"outcome":"error","message":"Command failed: docker inspect --type image ./Dockerfile","description":"An error occurred building the container."}
```

Can you please alleviate this check to support this use case? I would be very grateful.