aspire doctor should check for Docker buildx

## Description

`aspire doctor` checks for Docker/Podman presence and version, but does not check for the `docker buildx` plugin. When deploying with any target that requires building container images (Docker Compose, Kubernetes/Helm, etc.), the image build step fails with a confusing error:

```
Container runtime 'Docker' is not running or is unhealthy. Cannot build container image.
```

The actual issue is that `docker buildx version` returns exit code 1 because the buildx plugin is not installed, but the error message suggests Docker itself is unhealthy.

## Expected behavior

1. `aspire doctor` should check for `docker buildx` and report if it's missing
2. The image build step error message should distinguish between "Docker not running" and "buildx plugin not installed" — e.g. `Docker buildx plugin is not installed. Install it with 'apt-get install docker-buildx' or see https://docs.docker.com/buildx/install/`

## Reproduction

On Ubuntu 24.04 with `docker.io` installed (no buildx):

```bash
apt-get install -y docker.io # does NOT include buildx
aspire deploy # fails at image build step with misleading error
apt-get install -y docker-buildx # fixes the issue
```

This affects any deployment target that builds container images — Docker Compose, Kubernetes/Helm, etc.

## Additional context

Discovered while testing PR #16097 (`aspire destroy`) on a fresh Ubuntu 24.04 DigitalOcean VM.