Fix Dockerfile for NGINX on Cloud Run example (GoogleCloudPlatform#1415)

* Fix Dockerfile for NGINX example

The current Dockerfile for the react-nginx-cloud-run example substitutes the `$PORT` template variable at build-time, instead of run-time. This appears to work *some of the time*. However, this does not match the Cloud Run docs that specify that the server must listen on the provided `$PORT`, which I presume can be set to something other than 8080 at run-time.

* copy-edit

Co-authored-by: Todd Kopriva <43478937+ToddKopriva@users.noreply.github.com>

tutorials/deploy-react-nginx-cloud-run/index.md

@@ -66,7 +66,9 @@ Create a file in the root of the project named `nginx.conf` and add the followin
          
     }
 
-By creating this file, you provide the `$PORT` environment variable that Cloud Run expects your application to listen on. 
+By creating this file, you provide the `$PORT` environment variable that Cloud Run expects your application to listen on. The `$PORT`
+environment variable will be provided by the `docker run` command, so the Nginx server configuration template needs to have its environment
+variables substituted at run time, not at build time.
 
 This file customizes Nginx so that `react-router-dom` always responds with the proper route. This configuarion enables gzip
 compression, which makes the web application lightweight and fast. 
@@ -85,12 +87,14 @@ Create a file named `Dockerfile` in the root folder of the project and paste the
     # server environment
     FROM nginx:alpine
     COPY nginx.conf /etc/nginx/conf.d/configfile.template
+
+    COPY --from=react-build /app/build /usr/share/nginx/html
+
     ENV PORT 8080
     ENV HOST 0.0.0.0
-    RUN sh -c "envsubst '\$PORT'  < /etc/nginx/conf.d/configfile.template > /etc/nginx/conf.d/default.conf"
-    COPY --from=react-build /app/build /usr/share/nginx/html
     EXPOSE 8080
-    CMD ["nginx", "-g", "daemon off;"]
+    CMD sh -c "envsubst '\$PORT' < /etc/nginx/conf.d/configfile.template > /etc/nginx/conf.d/default.conf && nginx -g 'daemon off;'"
+
 
 This configuration defines two environments: one where the web application is built and one where the web application 
 will run.