More consistent use of the configured local port

Recent update in template-application-flask[1] along similar lines.

If someone wants to simultaneously run regular and "mock production"
instances, they can update the ports to what they want. By default
having everything use the same port locally seems sensible.

[1] navapbc/template-application-flask@ad74568

Author: doshitan

template/{{app_name}}/README.md.jinja

@@ -89,7 +89,7 @@ Once you've completed the setup steps above, you can run the site natively or wi
 To run within a container:
 
 1. `make start-container`
-1. Then visit http://localhost:3100
+1. Then visit http://localhost:{{ app_local_port }}
 
 To run natively:
 

template/{{app_name}}/docker-compose.mock-production.yml.jinja

@@ -34,4 +34,4 @@ services:
       - DISABLE_DATABASE_ENVIRONMENT_CHECK=1
       - AWS_DEFAULT_REGION=us-east-1
     ports:
-      - 3200:{{ app_local_port }}
+      - {{ app_local_port }}:{{ app_local_port }}

template/{{app_name}}/docker-compose.yml.jinja

@@ -29,7 +29,7 @@ services:
       - RAILS_BINDING=0.0.0.0
       - RAILS_ENV=development
     ports:
-      - 3100:{{ app_local_port }}
+      - {{ app_local_port }}:{{ app_local_port }}
     volumes:
       - .:/rails
       # Use named volumes for directories that the container should use the guest