Changed host port from 80 to 4000

The host port was set to 80, which was both inconsistent with the previous tutorial and also causing issues when following the tutorial on Windows.

For reference, here is the error when attempting to run a container on port 80:

C:\Program Files\Docker\Docker\Resources\bin\docker.exe: Error response from daemon: driver failed programming external connectivity on endpoint focused_euler (079893ea98df78bf7043f86072263317be106cbb7f4a3fcde23c2ef19ad39a01): Error starting userland proxy: Bind for 0.0.0.0:80: unexpected error Permission denied.
ERRO[0000] error waiting for container: context canceled

get-started/part3.md

@@ -26,8 +26,8 @@ Toolbox](/toolbox/overview.md).
 shared image here.
 
 - Be sure your image works as a deployed container. Run this command,
-slotting in your info for `username`, `repo`, and `tag`: `docker run -p 80:80
-username/repo:tag`, then visit `http://localhost/`.
+slotting in your info for `username`, `repo`, and `tag`: `docker run -p 4000:80
+username/repo:tag`, then visit `http://localhost:4000/`.
 
 ## Introduction
 
@@ -84,7 +84,7 @@ services:
       restart_policy:
         condition: on-failure
     ports:
-      - "80:80"
+      - "4000:80"
     networks:
       - webnet
 networks:
@@ -101,7 +101,7 @@ This `docker-compose.yml` file tells Docker to do the following:
 
 - Immediately restart containers if one fails.
 
-- Map port 80 on the host to `web`'s port 80.
+- Map port 4000 on the host to `web`'s port 80.
 
 - Instruct `web`'s containers to share port 80 via a load-balanced network
   called `webnet`. (Internally, the containers themselves publish to