Parametrize CUDA_VERSION in Dockerfile

Standardized CUDA_VERSION as a global ARG to ensure consistency and facilitate version updates across the Dockerfile. This change allows the CUDA version to be defined once at the beginning and reused, reducing the chance of mismatched versions and easing maintenance when changing CUDA versions. It further streamlines the build process for potential multi-stage builds with varying CUDA dependencies.

Refs #nvidia-update

Dockerfile-cuda

 # syntax=docker/dockerfile:1
+ARG CUDA_VERSION=12.3.2
 
 ######## WebUI frontend ########
 FROM node:21-alpine3.19 as build
@@ -23,8 +24,8 @@ RUN npm run build
 #RUN OPENWEBUI_CPU_TARGET="cpu_avx2" sh gen_linux.sh
 
 ######## CUDA WebUI backend ########
-ARG CUDA_VERSION=12.3.2
-FROM --platform=linux/amd64 nvidia/cuda:$CUDA_VERSION-devel-ubuntu22.04 AS cuda-build-amd64
+FROM --platform=linux/amd64 nvidia/cuda:"$CUDA_VERSION"-devel-ubuntu22.04 AS cuda-build-amd64
+
 # Set environment variables for NVIDIA Container Toolkit
 ENV LD_LIBRARY_PATH=/usr/local/nvidia/lib:/usr/local/nvidia/lib64 \
     NVIDIA_DRIVER_CAPABILITIES=all \