fix: Replace COPY --from=rootconfig with direct nuget.config copy

Docker additional_contexts with --from=rootconfig is not supported by Kaniko.
Changed to copy nuget.config directly from build context root.

- CI: pipeline already copies nuget.config to submodule root
- Docker Compose: users must run ./prepare-build.sh first

src/Nocr.TelegramListener.Host/Dockerfile

@@ -16,8 +16,11 @@ RUN apt-get update && \
     update-ca-certificates && \
     rm -rf /var/lib/apt/lists/*
 
-# Copy nuget.config from parent repository (via additional_contexts)
-COPY --from=rootconfig nuget.config /root/.nuget/NuGet/NuGet.Config
+# Copy nuget.config from build context root
+# Must be copied to submodule root before building:
+# - CI: pipeline copies it with "cp nuget.config telegram-listener/"
+# - Docker Compose: uses additional_contexts but file must also be copied
+COPY nuget.config /root/.nuget/NuGet/NuGet.Config
 
 COPY . .