Bump dotnet/sdk from 8.0 to 9.0 in /docker

Bumps dotnet/sdk from 8.0 to 9.0.

---
updated-dependencies:
- dependency-name: dotnet/sdk
  dependency-version: '9.0'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

docker/file-service.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 FileService/FileService.Core/FileService.Core.csproj FileService.Core/