diff --git a/modules/ContainerImages.nix b/modules/ContainerImages.nix
index bac088d..30a0460 100644
--- a/modules/ContainerImages.nix
+++ b/modules/ContainerImages.nix
@@ -12,6 +12,17 @@ in
 {
   # Configure ContainerImages.
   # This creates file `/etc/ContainerImages/dockerhub-p1st`.
+  #
+  # `config.json` is created by running `docker login` for the correct user.
+  # It has the following content
+  #   {
+  #   	"auths": {
+  #   		"https://index.docker.io/v1/": {
+  #   			"auth": "REPLACE-ME-WITH-REAL-VALUE"
+  #   		}
+  #   	}
+  #   }
+
   deployment.keys."dockerhub-${dockerHubUsername}" = {
     # mkdir secrets/dockerhub-p1st
     # sudo install -m600 /dev/stdin secrets/dockerhub-p1st/config.json