add reference to upstream launchpad issue

Author: johnstcn

images/base/ubuntu.Dockerfile

@@ -43,6 +43,7 @@ RUN apt-get update && \
 # Note: due to a dependency issue with the armv7 `software-properties-common` package,
 # we can't use `add-apt-repository` here. Instead, we'll add the Git PPA
 # manually. TODO: remove this workaround when the issue is resolved.
+# Ref: https://bugs.launchpad.net/cloud-images/+bug/2091619
 COPY git-core-ubuntu-ppa-noble.sources /etc/apt/sources.list.d/git-core-ubuntu-ppa-noble.sources
 RUN apt-get update && \
     apt-get install --yes git \