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

Bumps dotnet/sdk from 8.0-alpine to 9.0-alpine.

---
updated-dependencies:
- dependency-name: dotnet/sdk
  dependency-version: 9.0-alpine
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
 WORKDIR /src
 
 # Copy project files