fix: added comment for temporary workaround for .devcontainer build (#213)

Author: Akanshu-2u

.devcontainer/Dockerfile

@@ -1,3 +1,21 @@
+# TEMPORARY WORKAROUND:
+# This removes the legacy Yarn APT repository to unblock Codespaces pre-builds
+# failing with:
+#   GPG error: https://dl.yarnpkg.com/debian
+#   stable InRelease: The following signatures were invalid:
+#   EXPKEYSIG 23E7166788B63E1E Yarn Packaging <yarn@dan.cx>
+#
+# Root cause: expired/invalid Yarn signing key in the default Microsoft
+# devcontainer base image causing `apt-get update` to fail and abort
+# devcontainer feature installation.
+#
+# Tracking:
+# - https://2u-internal.atlassian.net/browse/BOMS-394
+# - https://2u-internal.atlassian.net/browse/BOMS-402
+#
+# IMPORTANT: Revert this change after confirming the upstream image no longer
+# includes the broken Yarn APT configuration.
+
 FROM mcr.microsoft.com/devcontainers/universal:latest
 
 # Fix: remove legacy Yarn apt repo that can break apt-get update with EXPKEYSIG