[libc++][NFC] Improve documentation for running BuildKite agent locally

diff --git a/libcxx/utils/ci/Dockerfile b/libcxx/utils/ci/Dockerfile
index aa719d8..4de0c80 100644 (file)
--- a/libcxx/utils/ci/Dockerfile
+++ b/libcxx/utils/ci/Dockerfile
@@ -15,7 +15,9 @@
 #   $ docker run --env-file <secrets> -it $(docker build -q libcxx/utils/ci)
 #
 # The environment variables in `<secrets>` should be the ones necessary
-# to run a BuildKite agent.
+# to run a BuildKite agent:
+#
+#   BUILDKITE_AGENT_TOKEN=<token>
 #
 # If you're only looking to run the Docker image locally for debugging a
 # build bot, see the `run-buildbot-container` script located in this directory.