docker: make CMAKE_CUDA_ARCHITECTURES configurable via build arg (#625)

Expose CMAKE_CUDA_ARCHITECTURES as a Docker build ARG so users can
customize CUDA architectures via --build-arg without editing the
Dockerfile.

- convert hardcoded ENV to ARG with default, feeding into ENV
- replace silent fallback defaults (:-) in scripts with :? guards
  to fail fast if the env var is missing
- add usage example to Dockerfile header

Follow up to: #624

https://claude.ai/code/session_01EWiUe7jNABX7Uz95dUGJqK

Co-authored-by: Claude <noreply@anthropic.com>

docker/unified/install-sd.sh

@@ -33,7 +33,7 @@ if [ "$BACKEND" = "cuda" ]; then
     CMAKE_FLAGS+=(
         -DGGML_CUDA=ON
         -DGGML_VULKAN=OFF
-        "-DCMAKE_CUDA_ARCHITECTURES=${CMAKE_CUDA_ARCHITECTURES:-60;61;75;86;89}"
+        "-DCMAKE_CUDA_ARCHITECTURES=${CMAKE_CUDA_ARCHITECTURES:?CMAKE_CUDA_ARCHITECTURES must be set}"
         "-DCMAKE_CUDA_FLAGS=-allow-unsupported-compiler"
         "-DCMAKE_EXE_LINKER_FLAGS=-Wl,-rpath-link,/usr/local/cuda/lib64/stubs -lcuda"
         "-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-rpath-link,/usr/local/cuda/lib64/stubs -lcuda"