Allow BUILDKIT_HOST to build Docker Container Actions in Kubernetes mode

What would you like added?

Allow the kubernetes mode GHA runner to run a remote build for Docker Container Actions if a BUILDKIT_HOST and image registry is provided.

Why is this needed?

Today, when passing a Docker Container Actions such as this one to a self hosting GHA in kubernetes mode, there will be a hardcoded error:

Error: Error: Building container actions is not currently supported
Error: Process completed with exit code 1.
Error: Executing the custom container implementation failed. Please contact your self hosted runner administrator.

It makes sense as in this case, by default there is no privilege to run docker build in local without dind.
But in the case the user already has configured a proper remote buildkit, buildx could simply execute the build and override the container when creating the pod.

Additional context

This is based on this community discussion
https://github.com/orgs/community/discussions/172177

Start with the Kubernetes-mode runner path for Docker Container Actions and trace how BUILDKIT_HOST and an image registry are handled. Check the existing implementation and tests for the hardcoded unsupported error; done means a configured remote BuildKit can build the action and the resulting container can be used to create the pod.