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 \
diff --git a/build/docker/ubuntu-jammy/Dockerfile b/build/docker/ubuntu-jammy/Dockerfile
index 220fa2b..8f66a56 100644
--- a/build/docker/ubuntu-jammy/Dockerfile
+++ b/build/docker/ubuntu-jammy/Dockerfile
@@ -51,12 +51,6 @@
   curl https://storage.googleapis.com/download.dartlang.org/linux/debian/dart_stable.list > \
   /etc/apt/sources.list.d/dart_stable.list
 
-# dotnet (netcore)
-RUN curl https://packages.microsoft.com/keys/microsoft.asc | gpg --dearmor > /etc/apt/trusted.gpg.d/microsoft.gpg && \
-  wget -q -O /etc/apt/sources.list.d/microsoft-prod.list https://packages.microsoft.com/config/ubuntu/22.04/prod.list && \
-  chown root:root /etc/apt/trusted.gpg.d/microsoft.gpg && \
-  chown root:root /etc/apt/sources.list.d/microsoft-prod.list
-
 # node.js
 RUN curl -sL https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - && \
   echo "deb https://deb.nodesource.com/node_16.x focal main" | tee /etc/apt/sources.list.d/nodesource.list