diff --git a/build/docker/ubuntu-focal/Dockerfile b/build/docker/ubuntu-focal/Dockerfile
index 50bb64c..e0244fc 100644
--- a/build/docker/ubuntu-focal/Dockerfile
+++ b/build/docker/ubuntu-focal/Dockerfile
@@ -137,6 +137,7 @@
       dart=$DART_VERSION
 ENV PATH /usr/lib/dart/bin:$PATH
 
+# Because Ubuntu 20.04 reaches end of life in April [actually May] 2025, Microsoft has decided not to support .NET 9 on Ubuntu 20.04.
 RUN apt-get install -y --no-install-recommends \
       `# dotnet core dependencies` \
       dotnet-sdk-8.0 \