Clarify .gitconfig usage in Makefile shell target

Added comment explaining that this file is mounted as /root/.gitconfig
by the 'shell' target in the Makefile to provide better context for
how this configuration is used in the build process.

Signed-off-by: Tiger Kaovilai <tkaovila@redhat.com>

Author: kaovilai

.gitconfig

@@ -1,6 +1,8 @@
 # Git configuration for containerized builds
 # 
-# This file fixes VCS stamping errors in Podman container environments.
+# This file is mounted as /root/.gitconfig in the container by the 'shell'
+# target in the Makefile to fix VCS stamping errors in Podman environments.
+#
 # When Docker/Podman mounts the repository, the mount point directory
 # (/github.com/vmware-tanzu/velero) is owned by root:root but the files
 # inside are owned by the mapped user. This ownership mismatch triggers