docs: add description for new variables

Signed-off-by: Felipe Zipitria <[email protected]>

README.md

@@ -208,6 +208,10 @@ These variables are common to image variants and will set defaults based on the
 
 | Name     | Description|
 | -------- | ------------------------------------------------------------------- |
+| CORS_HEADER_403_ALLOW_ORIGIN | When generating a 403 response from the server, the origin can access this response using the [Access-Control-Allow-Origin](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin) header (Default: `"*"`). |
+| CORS_HEADER_403_ALLOW_METHODS | When generating a 403 response from the server, the methods that will be sent in the header [Access-Control-Request-Method](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Request-Method) . Default: `"GET, POST, PUT, DELETE, OPTIONS"` |
+| CORS_HEADER_403_CONTENT_TYPE | When generating a 403 response from the server, the content-type of the response. Default: (`"text/plain"`) |
+| CORS_HEADER_403_MAX_AGE | When generating a 403 response from the server, the content of the [Access-Control-Max-Age](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age) header. (Default: `3600`) |
 | DNS_SERVER  | A string indicating the name servers used to resolve names of upstream servers into addresses. For localhost backend this value should not be defined (Default: _not defined_) |
 | KEEPALIVE_TIMEOUT  | Number of seconds for a keep-alive client connection to stay open on the server side (Default: `60s`) |
 | NGINX_ALWAYS_TLS_REDIRECT | A string value indicating if http should redirect to https (Allowed values: `on`, `off`. Default: `off`) |