[docker] Bump dotnet/sdk from 8.0 to 9.0 in /src

Bumps dotnet/sdk from 8.0 to 9.0.

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

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

src/Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/sdk:8.0 AS builder
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS builder
 
 WORKDIR /src
 COPY ./ /src