deps(docker)(deps): bump dotnet/sdk in /backend

Bumps dotnet/sdk from 9.0-alpine to 10.0-alpine.

---
updated-dependencies:
- dependency-name: dotnet/sdk
  dependency-version: 10.0-alpine
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/Dockerfile

@@ -1,5 +1,5 @@
 # Simplified Dockerfile for ModernAPI Backend
-FROM mcr.microsoft.com/dotnet/sdk:9.0-alpine AS build
+FROM mcr.microsoft.com/dotnet/sdk:10.0-alpine AS build
 WORKDIR /src
 
 # Copy only production projects (exclude tests for faster builds)

backend/Dockerfile.original.backup

@@ -1,5 +1,5 @@
 # Production-ready Dockerfile for ModernAPI Backend
-FROM mcr.microsoft.com/dotnet/sdk:9.0-alpine AS build
+FROM mcr.microsoft.com/dotnet/sdk:10.0-alpine AS build
 WORKDIR /src
 
 # Copy csproj files and restore dependencies (better layer caching)