Add `cuda13` as a supported build architecture, targeting the
`ghcr.io/ggml-org/llama.cpp:server-cuda13` upstream base image.
The `server-cuda13` image ships with CUDA 13 libraries, providing
improved performance on recent NVIDIA hardware compared to the existing
`server-cuda` (CUDA 12) image. Users with newer GPUs (e.g., RTX
50-series) benefit from reduced model load latency and higher token
throughput.
- Add `cuda13` to the allowed architectures list in
`docker/build-container.sh`
- Add `cuda13` to the CI matrix in `.github/workflows/containers.yml` so
the container is built and pushed automatically
