Fix Docker build: specify net8.0 framework for publish

The project multi-targets net8.0 and net8.0-windows, so dotnet publish
requires an explicit --framework to disambiguate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -35,6 +35,7 @@ COPY LICENSE LICENSE
 
 RUN dotnet publish src/NzbDrone/Prowlarr.csproj \
       -c Release \
+      -f net8.0 \
       -r "$(cat /tmp/rid)" \
       --self-contained \
       --no-restore \