Tighten timeout and add comment

ci/docker-compose.yml

@@ -51,10 +51,12 @@ services:
     build:
       context: .
       dockerfile: js-tests-browser.Dockerfile
+    # The test runner starts listening on the port slightly before it starts
+    # acceptions requests. Waiting a second makes this work more reliably.
     command: |
       bash -c "
         ./wait-for-it.sh -t 30 js-tests:9876
-        sleep 5
+        sleep 1
         firefox js-tests:9876 --headless "
 
 volumes: