Set ImageOS environment variable in runner images (actions#2878)

Co-authored-by: Nikola Jokic <jokicnikola07@gmail.com>

images/Dockerfile

@@ -37,6 +37,7 @@ FROM mcr.microsoft.com/dotnet/runtime-deps:6.0-jammy
 ENV DEBIAN_FRONTEND=noninteractive
 ENV RUNNER_MANUALLY_TRAP_SIG=1
 ENV ACTIONS_RUNNER_PRINT_LOG_TO_STDOUT=1
+ENV ImageOS=ubuntu22
 
 RUN apt-get update -y \
     && apt-get install -y --no-install-recommends \