diff --git a/ci/images/docker-lib.sh b/ci/images/docker-lib.sh
index d16a9a67fc2..c5cc985087e 100644
--- a/ci/images/docker-lib.sh
+++ b/ci/images/docker-lib.sh
@@ -1,3 +1,5 @@
+# Ref: https://github.com/concourse/docker-image-resource/blob/master/assets/common.sh
+
 sanitize_cgroups() {
   mkdir -p /sys/fs/cgroup
   mountpoint -q /sys/fs/cgroup || \