Add comment to prod.sh entrypoint

The equivalent comment is already in the Dockerfile, but this is
included now with the entrypoint script for the benefit of future
us coming across it again.

docker/entrypoints/prod.sh

@@ -1,5 +1,11 @@
 #!/bin/bash
 
+# Note: this is set as the default CMD in the Dockerfile
+# In production, dokku extracts the Procfile and uses the commands defined
+# there as the entrypoints, so this script is ignored in production. It is
+# used only if we're running the production service with docker compose
+# https://dokku.com/docs/processes/process-management/#procfile
+
 set -euo pipefail
 
 just run bot