fix: align runtime image .NET to the app (9.0)

The runtime image based on aspnet:10.0 provides only .NET 10.x, but the
publish step builds framework-dependent against SDK 9.0, so the app requires
Microsoft.NETCore.App 9.0.0 and crashes on start under 10.x.

Pin the runtime base to aspnet:9.0 to match the SDK 9.0 publish.

Dockerfile.runtime

@@ -22,7 +22,10 @@ RUN apt-get update \
  && rm -rf /var/lib/apt/lists/*
 
 # ── Runtime stage ─────────────────────────────────────────────────────────────
-FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/aspnet:10.0
+# .NET 9 runtime: matches the SDK 9.0 publish step (framework-dependent), so the
+# app's required Microsoft.NETCore.App 9.0 is present. Keep in lockstep with the
+# `mcr.microsoft.com/dotnet/sdk` major in .woodpecker/*.yaml and the Makefile.
+FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/aspnet:9.0
 
 # FFmpeg and the native deps required by SkiaSharp and fontconfig.
 RUN apt-get update \