diff --git a/serving/samples/helloworld-dart/Dockerfile b/serving/samples/helloworld-dart/Dockerfile
index b9517974e..6066d9c4d 100644
--- a/serving/samples/helloworld-dart/Dockerfile
+++ b/serving/samples/helloworld-dart/Dockerfile
@@ -1 +1,7 @@
+# Use Google's official Dart image.
+# https://hub.docker.com/r/google/dart-runtime/
 FROM google/dart-runtime
+
+# Configure and document the service HTTP port.
+ENV PORT 8080
+EXPOSE $PORT
diff --git a/serving/samples/helloworld-dart/README.md b/serving/samples/helloworld-dart/README.md
index d1067b5ec..c1fcd2724 100644
--- a/serving/samples/helloworld-dart/README.md
+++ b/serving/samples/helloworld-dart/README.md
@@ -71,7 +71,13 @@ created using the following instructions.
    dockerizing your applications, for dart apps this can be done as follows:
 
     ```Dockerfile
+    # Use Google's official Dart image.
+    # https://hub.docker.com/r/google/dart-runtime/
     FROM google/dart-runtime
+
+    # Configure and document the service HTTP port.
+    ENV PORT 8080
+    EXPOSE $PORT
     ```
 
 5. Create a new file, `service.yaml` and copy the following service definition