chore(docs): add composer install instructions (#1789)

Co-authored-by: Katie McLaughlin <[email protected]>

Author: bshaffer

run/helloworld/README.md

@@ -3,3 +3,20 @@
 This sample demonstrates how to deploy a **Hello World** application to Cloud Run.
 
 **View the [full tutorial](https://cloud.google.com/run/docs/quickstarts/build-and-deploy/deploy-php-service)**
+
+# Adding Composer
+
+To add composer to this example, add the following to the minimum `Dockerfile`
+included in this sample:
+
+```
+# composer prefers to use libzip and requires git for dev dependencies
+RUN apt-get update && apt-get install git libzip-dev -y
+
+# RUN docker-php-ext-configure zip --with-libzip
+RUN docker-php-ext-install zip
+
+# Install compoesr dependencies
+COPY --from=composer /usr/bin/composer /usr/bin/composer
+RUN composer install
+```