CORS error when integrating with REST API

### Preconditions

1. Magento `2.4.2`
2. NGINX image: `magento/magento-cloud-docker-nginx:1.19-1.2.2`

### Steps to reproduce

1. Setup a sample React App.
2. Use Magento REST API ([integration/admin/token](https://magento.redoc.ly/2.4.2-admin/tag/integrationadmintoken)) for authentication.
3. When calling na API, I get the CORS error.

### Expected result

For the NGINX image I am able to define an environment variable:
```
ENV REST_API_ORIGINS localhost:3000|staging.cloud
```
and then in the `vhost.conf` we add required headers:

```
# PHP entry point for main application
location ~ ^/(index|get|static|errors/report|errors/404|errors/503|health_check)\.php$ {
set $cors_origin "";
if ($http_origin ~ '^https?://(!ORIGINS!)$') {
set $cors_origin $http_origin;
}
if ($request_method = 'OPTIONS') {
add_header 'Access-Control-Allow-Origin' $cors_origin always;
add_header 'Access-Control-Allow-Methods' 'POST, GET, OPTIONS' always;
add_header 'Access-Control-Allow-Headers' 'Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With,X-Cache-Hash' always;
### Tell client that this pre-flight info is valid for 20 days
add_header 'Access-Control-Max-Age' 1728000;
add_header 'Content-Type' 'text/plain charset=UTF-8';
add_header 'Content-Length' 0;
return 204;
}
add_header 'Access-Control-Allow-Origin' $cors_origin always;
...
}
```

### Actual result

Start by reproducing the call from the sample React app against the Magento REST API using the stated Magento and NGINX image versions. Then inspect the NGINX image's vhost.conf configuration and compare its response headers with the issue's example; done means the API request no longer fails with a CORS error.

Written by the indexing model from the issue text.