diff --git a/dev/ci/docker_linux/Gemfile b/dev/ci/docker_linux/Gemfile
index 9b041f8..9fdb92a 100644
--- a/dev/ci/docker_linux/Gemfile
+++ b/dev/ci/docker_linux/Gemfile
@@ -1,3 +1,6 @@
+# When editing this file, update the comment starting "# Gemfile last edited"
+# in dev/ci/docker_linux/Dockerfile to retrigger a Docker build on Cirrus.
+
 source "https://rubygems.org"
 
 # Install fastlane which is used on Linux to build and deploy Android